Drop usage of deprecated Gtk+ properties

These properties are now deprecated:

- Gtk.Misc.xalign
- Gtk.Misc.yalign
editable-entry: Drop usage of deprecated Gtk.Alignment
wizard-source: Drop usage of deprecated Gtk.Alignment
Port away from LTR-specific deprecated margin props

LTR-specific horizontal margin properties of Gtk.Widget have now been
deprecated in favor of generic ones.