Glade files: Now all windows have a 12 pixel border as per the HIG.

* For GtkWindows, set the border-width to 12, unless it needs a
  menu, in which case the window should have 0 border-width,
  and an inner GtkBox, a sub-GtkBox and GtkButtonBox should have
  a border width of 6, allowing the menu to fit snugly into the window edge.
  Using just a border-width of 12 on both the GtkBox and the
  GtkButtonBox would leave 24 pixels between them.
* For GtkDialog, set the border-width to 6 and give the GtkBox in
  the content area another border-width of 6, because the
  GtkButtonBox in the action area seems to have this already too.

  I do not know why GtkDialogs do not just have a border-width of
  12, and a VBox spacing of 12.
Relationships Overview window: HIG corrections.

This is now a GtkWindow, rather than a GtkDialog, allowing us
to arrange things so that the menu is snugly against the edge.
Relationships Overview Window: Make this into an ApplicationWindow.

* data/ui/developer/window_relationships_overview.glade:
  Change this from GtkWindow to GtkApplicationWindow.
* mode_design/relationships_overview/window_relationships_overview.cc:
  Change the print and page-setup actions to be in the standard
  win. group.
* glom/frame_glom.cc: Add the window to the GtkApplication.
* glom/application.cc: on_startup() : Add a Ctrl P acelerator.