Glade UI Files: Remove some unnecessary frames.

* data/ui/developer/*.glade: Remove parent GtkFrames and GtkAlignements
  if there is only one, because no grouping is necessary.
  This was probably necessary once because some older version of the
  GNOME theme did not show window titles for modal dialogs, but that
  does not seem to be the case anymore.
Layout window: Fix some packing.
Glade files: Minor improvements
Tests: Fix make check after a class rename.
Replace an error dialog in Glade with code that uses MessageDialog.
Glade files: Many small HIG improvements.

Mostly increasing the horizontal space between widgets from 6 to 12,
and increasing the vertical space between groups of widgets to 18.
See https://developer.gnome.org/hig-book/unstable/design-window.html.en#layout-dialogs