Clean up the docs for GApplication and related classes.
I'm no longer writing documentation for the structure type of classes
and interfaces. See https://bugzilla.gnome.org/show_bug.cgi?id=665926
for discussin on the correct way forward on this point.
Also: stop putting gtk-doc comments in installed headers.
This is an interface to represent GSimpleActionGroup-like objects (ie:
those GActionGroups that operate by containing a number of named GAction
instances).