applet: Fix the Bluetooth status icon not appearing

Port the "connected" and "connectable" statuses to
BlueZ 5/GNOME 3.10. The underlying API was changed, but
our code wasn't.

https://bugzilla.gnome.org/show_bug.cgi?id=708479
applet: Use g_clear_object()
lib: Correct error reporting for connect methods

And make sure to handle cancellations correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=708477
lib: Fix double-free when finishing connection

The proxy was already unref'ed when we started the
connection/disconnection so the only places that should hold a reference
are the tree model, and the operation itself.

Fixes the bugs reported in:
http://retrace.fedoraproject.org/faf/reports/205937/

https://bugzilla.gnome.org/show_bug.cgi?id=708477