Fix button order in wxGTK wxMessageDialog and wxStdDialogButtonSizer.

Fix button order in wxGTK wxMessageDialog and wxStdDialogButtonSizer.

The order of buttons in wxGTK wxMessageDialog was accidentally broken since
r55482 and wxStdDialogButtonSizer implementation for wxGTK was subsequently
modified to work in the same way as the broken version of wxMessageDialog.

Fix wxMessageDialog and remove the "compatibility" workarounds from
wxStdDialogButtonSizer to restore correct behaviour in both places.

Closes #12565.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65812 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775