Gtk::Application punted to gtkmm 3.2

[Murray Cumming <murrayc murrayc com>]

We should freeze the gtkmm API in the next day or so, to be roughly in
sync with the GNOME 3.0 schedule. Gtk::Application still isn't ready so
I have removed it for now. Hopefully we can figure things out and add it
for gtkmm 3.2. This is the gtkmm bug about it:
http://bugzilla.gnome.org/show_bug.cgi?id=637445#c17

This commit removes it, which can be reverted later:
http://git.gnome.org/browse/gtkmm/commit/?id=83f5d5f1ab792ff823d5748265907be97e422aea

And the code is still in this branch:
http://git.gnome.org/browse/gtkmm/log/?h=gtkapplication

As far as I can tell nobody was using Gtk::Application in gtkmm yet,
though I guess people will start to want it once GNOME 3.0 is out.
 
-- 
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