Ported to GSettings

The port basically works, there are only two remaining issues:

1) at start-up time two critical messages are printed when trying to
set a value but I couldn't find out why, the message is the following:
g_source_get_context: assertion `!SOURCE_DESTROYED (source)' failed

2) The Nautilus port to GSettings is required in order to connect to the
correct "single click policy" key.