Make ad-hoc autoconnnect optional

Add a gsettings value to determine whether or not
ad-hoc networks should autoconnect.

This value will be ignored if the user explicitly connects
to a ad-hoc network, for the rest of the session. It will be taken
into account again, if the user explicitly disconnect from the
ad-hoc network.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Merge branch 'tchx84-optional-autoconnect-try2'