Use special code to identify custom colour in wxPG.

There is already defined a special code (wxPG_COLOUR_CUSTOM) to identify custom colour in wxColourProperty and wxSystemColourProperty but in practice item representing this colour is identified either by position (it is assumed it as always last item) or by fixed text label. This inconsistency makes the code unclear and can lead to problems (see workaround r76627).
Now, only wxPG_COLOUR_CUSTOM code is used to identify custom colour and hence "custom colour" item doesn't need to be the last item in the collection and doesn't need to have a fixed text label (like "Custom").


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77696 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775