Allow using custom labels for wxGenericMessageDialog buttons.

Allow using custom labels for wxGenericMessageDialog buttons.

Custom labels set for wxGenericMessageDialog buttons were simply ignored as it
used CreateSeparatedButtonSizer() to create the actual buttons which in turn
always used the standard labels.

Fix this by explicitly creating the buttons with custom labels if necessary.

This also fixes custom label support in wxGenericRichMessageDialog deriving
from this class.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65449 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775