The existing 'ExtensionStatusChanged' signal has a fixed set of parameters,
which means we cannot add additional state without an API break. Deprecate
it in favor of a new 'ExtensionStateChanged' signal which addresses this
issue by taking the full serialized extension as parameter.
https://bugzilla.gnome.org/show_bug.cgi?id=789852
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
