QtThreadedCanvas used to ignore all repaint events send to it by Qt in
order to avoid problems with threading. As a result, however, it would
become blank when docked/undocked, and display garbage in those of its
regions that some other widgets have moved over.

To fix this, the API of TGT's Painter class had to be slightly modified
and now mirrors that of Canvas with regard to painting-related
operations. The paint method has been made protected; it's supposed to
implement the painting logic and shouldn't be called directly. A public
repaint method has been introduced to let canvases notify their
associated painters that they need to be redrawn. The default
implementation simply calls paint() immediately; threaded painters,
however, override it to schedule render jobs that run in separate
threads.

All existing threaded painters have been updated to reduce the
visibility of their paint methods and provide public repaint functions.
Consequently, QtThreadedCanvas c...