Make Geary DBus-activatable. Bug 775956.

* org.gnome.Geary.service.in: create, and let it be installed by CMake.

* org.gnome.Geary.desktop.in: add `DBusActivatable=true`.

* geary-autostart.desktop.in: use `--gapplication-service` instead of
hidden.

* Change the way arguments are parsed:
  * Use the _command-line_ and _handle-local-options_ signals instead
    of _local-command-line_.
  * Remove the `--hidden` option (replaced by `--gapplication-service`)
  * Use VariantDict (which is a little cleaner)
  * Don't use global variables in Arg, but set them in
  the config instead.
  * We can no longer set a global summary due to the new option handling
  in GApplication. On IRC, I got the feedback that info like this should
  be going into a manpage.

* Since the QUIT-action can now be called without ever activating the
app, make the necessary changes to the `GearyController`:
  * `main_window` and `current_conversations` can be null.
  * use `pending_mailtos()` for the compose action as well.
  * Don't update the UNDO action if we're closing down.

* More instance variables prefixed with this (we're changing the lines
anyway, might as well do it properly).

* `Environment.set_prgname()` is already executed in GApplication.run(),
so no more need for `GearyApplication.PRGNAME`.

Signed-off-by: Niels De Graef <nielsdegraef@gmail.com>

desktop/org.gnome.Geary.service.in

@@ -0,0 +1,3 @@
+[D-BUS Service]
+Name=org.gnome.Geary
+Exec=@bindir@/geary" --gapplication-service