Add QWindow::startSystemMove and startSystemResize

This can be used to create custom client side window decorations.

Refactors the xcb implementation to use edges instead of corners and we now use
the last mouse position for `root_x` and `root_y` in the `_NET_WM_MOVERESIZE`
event. Touch has also been changed, so just pick a point that's currently being
pressed.

The workaround for QTBUG-69716 has now been moved to QSizeGrip, as the comment
in the bug report says that it should ideally be fixed at the widget level.

On Windows, we no longer abort when GetSystemMenu returns false. I assume this
code was added to check whether the window didn't have any decorations and not
resize in that case. However, since the point of this patch is to let windows
without native decorations resize/move, it makes most sense to remove the
check.

Adds a manual test, which calls QWindow::startSystemMove and startSystemResize
on touch and mouse events.

[ChangeLog][QtGui] Added API for starting interactive window resize and move
operations handled by the system.

Fixes: QTBUG-73011
Change-Id: I7e47a0b2cff182af71d3d479d6e3746f08ea30aa
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

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