Simplificate QWidget* assignment (new PostCreation method)

Also reducing the redundant m_qtWindow code, this could help to unify application of background style and default colors (for painting, that was not being enforced and could cause issues). Window style code is commented in the original, maybe it belongs to wxFrame.
PostCreation method name was taken from wxGTK, that has similar features.
Fix default widget fore/background colour
Implement missing wxQT helpers for QLabel

This now is needed to correctly paint the background, allows the static text / bitmap to handle events and signals, and maybe a better destruction process.