Improved support for Gtk Status icon

It should now have the same functionalities as the old Egg tray icon.
Fix UI play/pause/... buttons on reconnection

The code tried to detect when the connection status changed to update the status of the icons in the
GUI.

This fix the following bug:

 1. Sonata is running, connected to a MPD server
 2. the MPD server is suddenly shutdown and then restarted
 3. Sonata correctly reconnect to the MPD server, but the UI buttons were still deactivated
Fixed a TODO item \o/
Use TextView instead of Label for lyrics and album info
My first humble attempt to make sonata a little bit more friendly. If user modified his custom artwork location in display preferences the combobox still shos senseless 'SONG_DIR/custom'. This patch replace it with actual file name (preceded with 'SONG_DIR/').
Fix a bug Destroy dialog after all data is read from it. Otherwise artist_entry.get_text() returns empty string
Fix some string issue (% in song path name, empty artist)
Merge branch 'fix-ui-connection'
Merge branch 'refactor-tray-icon'
Fix a bug when we close the window. The window can now be hided
load only latest version of each plugin
Changes on file about.py: PEP8 compliant
Changes on file artwork.py: PEP8 compliant
Changes on file audioscrobbler.py: PEP8 compliant
Changes on file breadcrumbs.py: PEP8 compliant
Changes on file cli.py: PEP8 compliant
Changes on file config.py: PEP8 compliant
Changes on file consts.py: PEP8 compliant
Changes on file current.py: PEP8 compliant
Changes on file dbus_plugin.py: PEP8 compliant