Allow use of all wxPG colours.

wxPG-specific colours are added to the colour database when wxColourProperty is created (already existing colours are not overwritten by wxPG ones).
Original list of wxPG colours was restored.

Closes #15597.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76893 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775