0. extracted toolbar colour mapping stuff to its own function

0. extracted toolbar colour mapping stuff to its own function
1. wxToolBar adjusts the colours properly when the system colours change
2. wxImageList uses light grey as the transparent colour for the bitmaps

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@13558 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775