Fix manual selection of custom colour item for wxColourProperty and wxSystemColourProperty in wxPG.

If keyword 'custom' is entered into the edit field of wxColourProperty or wxSystemColourProperty (with wxPGEditor_ComboBox) then dialog box to query for custom colour should be invoked. Afterwards, when control value is updated, its selection index should be also updated respectively.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77738 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775