Create D-Bus api
Main Window: allow only one instance
new class DbusClient, rename self._app to self._application
Main Window: use new DbusClient class
Try to make dbus optional
single instance fallback
Main window: Make argument paths absolute.
Merge pull request #12 from ArseniyK/Issue_342

Make a path absolute before parsing arguments.
Merge pull request #11 from ArseniyK/dbus

Allow only one instance through DBus support or file.
Rename DBus implementation file.