socketservice: add an "active" property

socketservice: add an "active" property

We already have start, stop and is_active methods, but turning it
into a real property is useful for a few reasons:
 - it allows us to bind the property to an UI or a setting
 - it allows us to get notified when the state changes
 - it allows us to instantiate objects directly in the stopped state

https://bugzilla.gnome.org/show_bug.cgi?id=752089