fix some more PyGIDeprecationWarning exceptions (Gtk.FileChooserDialog())
port to new GtkSource.SearchContext API

GeditDocument.set_search_text() has been deprecated (it's already broken
in Gedit 3.10 and has been completely removed in 3.12); so use the new
GtkSource.SearchContext API if it is available.

Unfortunately this requires an API extension
(GeditDocument.set_search_context()) to work well, which is available
starting with Gedit 3.12 only - see bugs.gnome.org/726408.
Without this function there might be multiple search highlights in the
same document, with no way to clear them.
Currently I have no idea how this could be solved for 3.10.
Gedit 3.12 port: handle new GtkStack-based bottom panel
Gedit 3.12 port: use new filebrowser API if available

The "add_context_item" API for filebrowser plugin has been removed in Gedit
3.10. Starting with 3.12, a new API is available (extend_context_menu);
see bugs.gnome.org/725424.
use better way to detect whether filebrowser plugin is ready

Waiting for filebrowser messages to be registered on the bus avoids the
hack in do_update_state().
Gedit 3.12 port: use new app.extend_menu() API if available