Redone (generic) wxComboCtrl background painting and handling. The 'actual' wxWindow...

Redone (generic) wxComboCtrl background painting and handling. The 'actual' wxWindow background colour is now largely ignored and overridden to refer the text-area's background colour instead (as is usually the case with controls like this). Base 'transparent' background is now only painted when double-buffered rendering is required, and otherwise delegated to the system, as appropriate. This should significantly improve control's appearance and compliancy with GTK+ and OS X themes and custom backgrounds.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67255 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775