native GTK3 message dialogs

In LibreOffice 6.1, when the GTK3 backend is in use, the message dialogs are now native GTK3 message dialogs rather than vcl message dialogs using GTK theming.

So they are now true GTK dialogs and they look like...

while before they looked like...

We have 475 message dialogs directly instantiated in LibreOffice and 89 indirectly instantiated via GtkBuilder .ui files.

We've changed the format our dialog, etc. translations are stored in from our own legacy custom format to the more commonplace gettext .mo format. In the case of a message dialog described via a GtkBuilder .ui file, we now use GTK's own GtkBuilder to load the .ui, and GTK's own gettext integration to localize it from our new .mo format translations.