Move style updates in AppIcon._onStateChanged out

There is currently no simple way to inject into AppIcon's state change,
so an extension that wants to do this has to destroy/remove/update all
icons in the Shell (i.e. in the Dash, AllView, FrequentView) on enable()
and disable() after updating AppIcon.prototype._onStateChange, or the
extension must require a restart of the Shell.

To solve this issue, we rename _onStateChanged to _updateRunningStyle,
and connect the notify::state signal with an anonymous function that
calls _updateRunningStyle.
This extra function call should allow extensions to just extend the
updateRunningStyle function in the prototype.

https://bugzilla.gnome.org/show_bug.cgi?id=739497