location: Create Geoclue proxy asynchronously

https://bugzilla.gnome.org/show_bug.cgi?id=723684
location: Rename _onGeoclueAppeared to _connectToGeoclue

Rename _onGeoclueAppeared to _connectToGeoclue as in the following patch
we'll call it from other contexts then geoclue appearing.

https://bugzilla.gnome.org/show_bug.cgi?id=723684
location: Make _connectToGeoclue() reentrent

https://bugzilla.gnome.org/show_bug.cgi?id=723684
location: Provide a way to disable geolocation

Now that we are indicating 'geolocation in use' to user, we better also
provide at least a way to disable geolocation. Once this is in place, we
can provide slightly better controls rather than simply on/off switch.

https://bugzilla.gnome.org/show_bug.cgi?id=723684
location: Don't show menu if geoclue is totally incapable

If geoclue can't provide location at all (no GPS or network), don't
bother to show controls that don't serve any purpose.

https://bugzilla.gnome.org/show_bug.cgi?id=723684
location: React to changes in available accuracy level

GPS could be plugged in and out, network can go up and down and
therefore available accuracy level can also change.

https://bugzilla.gnome.org/show_bug.cgi?id=723684