It's unnecessary, and only adds visual noise; we have been fairly
inconsistent in the past, but the semi-colon-less version clearly
dominates in the code base.
https://bugzilla.gnome.org/show_bug.cgi?id=669355
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
