Widgets: enable scroll buttons to be placed anywhere on a QTabBar

If a tab bar contain more tabs than it can fit inside its geometry, it
will show two scroll buttons that lets the user scroll left or right.
From before, those buttons where hard coded to always be placed
together on the right side of the tab bar.

This patch will make it possible for the style to specify the exact
geometry of both scroll buttons. The reason for this is that 3rd party
styles has a specific need to place the "scroll left" button on the left
side, and the "scroll right" on the right side. Additionally, there is a
need to draw fade-out effects on tabs that end up half-way obscured by the
buttons. This can already be achieved by extending the tab tear concept to
include two tears/fade effects, one for each side of the tab bar.

Previous code in QTabBar that hard-coded scroll buttons and related
functionality will now be factored out to the style, and the base
style (QCommonStyle) will implement the old default logic of placing the
buttons together on the right side. Six new style enums will be added:

SE_TabBarScrollLeftButton: the rect of the left scroll button
SE_TabBarScrollRightButton: the rect of the right scroll button
SE_TabBarTearIndicatorLeft: the rect of the left tab tear
SE_TabBarTearIndicatorRight: the rect of the right tab tear
PE_IndicatorTabTearLeft: draw the left tab tear
PE_IndicatorTabTearRight: draw the right tab tear

Change-Id: I4cda05c2f7323de5cbd3ca071eb796085257c19b
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>

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