main: move state settings into gsettings

the window state was saved in a file named state.ini in the config dir.

Please note that when I was compiling the gschema I had the error:
Error on line 39 char 1: <enum id='org.gnome.totem.BvwAudioOutputType'> not (yet) defined.. --strict was specified; exiting.

But since I have this key on an already installed totem, I don't know what to thinK about this error...

Thanks !