extensionSystem: Add methods to enable/disable extensions

extensionSystem: Add methods to enable/disable extensions

Extensions are currently enabled or disabled by directly changing the
list in the 'enabled-extensions' GSettings key. As we will soon add
an overriding 'disabled-extensions' key as well, it makes sense to
offer explicit API for enabling/disabling to avoid duplicating the
logic.

For the corresponding D-Bus API, the methods were even mentioned in
the GSettings schema, albeit unimplemented until now.

https://bugzilla.gnome.org/show_bug.cgi?id=789852