Avoid showing the colour selection dialog twice in wxPropertyGrid.

Selecting "Custom" in a colour property cell resulted in the colour selection
dialog being shown twice if it was cancelled the first time.

Fix this by using wxPG_PROPERTY_SPECIFIC to indicate that the value is just
being queried and the user shouldn't be asked to enter it, as it is already in
the other places.

Closes #15543.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76593 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775