gedit3 port: assume that Gio and remote directories are always available

The "onlyLocalPathes" check was only necessary to remain compatible with
older Gnome versions which didn't have Gio.
gedit3 port: assume that window.get_message_bus() is always available

The hasattr() check was only necessary to remain compatible with older
Gedit versions which didn't have a message bus.
gedit3 port: port from GConf to GSettings
gedit3 port: simplify SearchQuery class

Default values for boolean search options are now stored solely in the
GSettings schema file.