Modernization:
  Replace unique by GtkApplication
  Replace GtkUIManager by GApplication/GMenu/GtkBuilder(?)
  Replace context menu by GMenu/GtkBuilder?
  Use the Undo interface of GtkSourceBuffer
  Use gsettings to store preferences

Some features that would be nice for a final Gobby 0.5 release. We could
perhaps do pre-releases with some of these missing before:

 - Add base preferences class with Preferences::Option definition to util/,
   inherit in core/, move ClosableFrame to util/
 - Show in tab and userlist when others are typing, using an icon
 - Typing sounds (libcanberra?)

These things need to be implemented in infinote, but again would be nice
to have:

 - Show other user's viewport position in the scrollbar, instead or in addition to cursor position
 - Show what tab another user currently is in (needs cross-session user management as planned for the authentication business)
