make sure dialogs can support alpha transparency sliders

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73639 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
support alpha in color dialog, see #14127

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73640 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775