Set colours and fonts for all elements of the generic wxSearchCtrl.

Set colours and fonts for all elements of the generic wxSearchCtrl.

Derive generic wxSearchCtrl from wxCompositeWindow<> to ensure that calls to
its Set{Fore,Back}groundColour() are propagated to all its children, otherwise
they applied only to the composite control itself but not e.g. its text part.

Also update the bitmaps when the background colour changes to ensure that it
appears as transparent.

Closes #13428.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68911 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775