Add spline-series example

Example shows using of points selection and adding light markers to plot.
User can select shapes for markers and light markers and set images for them.

[QtCharts][Example] Added Example

Task-number: QTBUG-89445
Task-number: QTBUG-92884
Change-Id: Id0c9a5a7b696b7a51804575f645578d06c717d4e
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
(cherry picked from commit 5070d1fc3ee4b63c8a04f1fdf9226b779a6cb167)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

3 files changed, 76 insertions, 0 deletions

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diff --git a/src/charts/doc/images/examples_pointsselectionandmarkers2.png b/src/charts/doc/images/examples_pointsselectionandmarkers2.png
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diff --git a/src/charts/doc/src/examples-pointsselectionandmarkers.qdoc b/src/charts/doc/src/examples-pointsselectionandmarkers.qdoc
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+/****************************************************************************
+**
+** Copyright (C) 2021 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:FDL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Free Documentation License Usage
+** Alternatively, this file may be used under the terms of the GNU Free
+** Documentation License version 1.3 as published by the Free Software
+** Foundation and appearing in the file included in the packaging of
+** this file. Please review the following information to ensure
+** the GNU Free Documentation License version 1.3 requirements
+** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+/*!
+    \example pointsselectionandmarkers
+    \title Light Markers Points Selection Example
+    \ingroup qtcharts_examples
+
+    \brief The example shows a usage of light markers
+     and points selection for the series.
+
+    \image examples_pointsselectionandmarkers1.png
+    \image examples_pointsselectionandmarkers2.png
+
+    \include examples-run.qdocinc
+
+    \section1 Light Markers Feature
+
+    We start with creating a series, filling it with the data, and setting up selection funcionality.
+    It's important not to set points visibility to \c true, because light markers functionality is
+    an independent feature and setting both would result in undesired behavior.
+    \snippet pointsselectionandmarkers/main.cpp 1
+
+    Then we create the \c {QChart}, the \c {QChartview} and the control widget with its layout to arrange customization elements.
+    \snippet pointsselectionandmarkers/main.cpp 2
+
+    The next step is creating elements of customization.
+    \snippet pointsselectionandmarkers/main.cpp 3
+
+    We create the label for the marker selection combobox and add fill the combobox with the items.
+    Also, we provide functionality to the combobox. Considering the implementation of the light markers
+    functionality, we need to make sure if displaying of unselected points is checked.
+    Switching the visibility of the light marker off is achieved by setting it to \c "empty" \c {QImage}.
+    If the user unchecks the displaying of unselected points and changes the light marker image, unselected points
+    have to remain invisible.
+    If checking isn't performed, new \c {QImage} will be set for light marker
+    and unselected points will be visible even though it has been switched off.
+    \snippet pointsselectionandmarkers/main.cpp 4
+
+    Almost the same procedure applies to the selected point light marker and line color. The only difference is
+    that there's no need to check visibility of unselected points as it doesn't affect the functionality.
+    \snippet pointsselectionandmarkers/main.cpp 5
+
+    A small difference comes with changing visibility of unselected points. As it was mentioned before,
+    making light markers invisible is achieved by setting them to "empty" \c {QImage}.
+    That is why, depending on checkbox state, selected point light marker is set to "empty" \c {QImage} or
+    to the light marker extracted from the current index of the corresponding combobox.
+    \snippet pointsselectionandmarkers/main.cpp 6
+
+    The final part is to arrange the elements, add all the widgets to the main widget, and set the main window size.
+    \snippet pointsselectionandmarkers/main.cpp 7
+*/