Gobjectify tags dialog.

NOTE: Not ready to merge. I do the pull request for discussing.

The main advantage is remove remove the gtk_dialog_run() that blocks all main loop and uses signals to respond to events.

Doubts: Is better use separate callbacks? (As is now, a callback to edit tags on menubar, other to lastfm, playlist, etc) or make a single callback that meets all your needs?.

Mm.. As PraghaBackend the dialog now save a copy of current Musicobject. Then you have to check the current song considering the file name. I think that in 99.99 % of cases is sufficient. There will be a problem?

TODO:
* Fix mem leaks. etc.
* Connect some signals. GtkEntry Icons, text changes, etc.
* Add artist, album, genre, completions. (Maybe do it directly on PraghaDatabase)
* Surely other things..