Added wxImage::Replace() (replaces one colour with another).
Toolbar tips no longer eats ana new colour. This might prevent
the wrong colour behaviour reported from some 8-bit visuals.
Minor fixes and test code.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@5122 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775