Avoid calling convertFromGLImage on a null image.

Task-number: QTBUG-34017

Change-Id: If51ef4b1f906677e26d14b5a92799097d18ac437
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Accessibility: simplify dockwidget accessible

This patch merges the two classes that were used to make
dock widgets accessible into one.
The title bar does not need to be represented by its own
accessible object.

In addition the buttons on the toolbar are now labelled.

Task-number: QTBUG-33946
Change-Id: Id90d8c09f15ed683e64dbe3f6ac55bca7a0b300f
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Windows: Fix tablet position in relative (mouse) mode.

When in "mouse" or "relative" mode with the pen position information
would not be calculated correctly resulting in a significant offset
between the tablet pen location and the mouse cursor location. Logic
was added to detect when the two were not in sync and use the mouse
location when this happens.

Change-Id: Icb7129e8cce186c0099953769e215649d9347c8e
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>