Replaced CUIButton::MarkNotSelected, MarkSelectedGray and MarkSelectedTechCategoryColor with an internal state to move the color selection reponsibility to the button user.

git-svn-id: svn://svn.code.sf.net/p/freeorion/code/trunk@7630 dbd5520b-6a0a-0410-a553-f54834df5b05