Fix stale bitmap deselection in memmory DC causing SIGSEGV (wxQT)

Seems that Qt can draw directly on a QPixmap but doesn't have get pixel / blit so it is using a QImage, that doesn't update the pixmap directly.
To reproduce and analyze, see the wxSearchCtrl::RenderCancelBitmap, that is not destroying the DC after using the generated bitmap, and crashing then as the bitmap (hence QPixmap) may be destroyed first.
If this is a common use case, it could be causing drawing issues everywhere, and the solution will not be simple as some logic will need to be moved to wxBitmap or DC,  using temporary conversions to QImage back and forth for bits methods.
Fix toolbar tool deletion incorrect return value in wxQT

This was causing an infinite loop in the related unit test
Fix toolbar parenting properly to avoid crashes

 * Removed logic to search the parent by @seandepagnier, and implemented wxFrame::SetToolBar (including removal to NULL)
 * Added missing AddChild that was causing SIGSEGV due toolbar not being deleted when the parent was destroyed
 * Adapted the code to follow current wxQT coding patterns (unifying QtToolBar -> GetHandle) and minor styling issues (space after if)