Add two new APIs, "launchExtensionPrefs" to let SweetTooth let the user
launch the extension preferences tool directly from the browser. To allow
SweetTooth to check if an extension can be configured, add a new key to
the 'metadata', 'hasPrefs', which is returned by the GetExtensionInfo/
ListExtensions DBus methods.
https://bugzilla.gnome.org/show_bug.cgi?id=668429
ExtensionUtils is a new module that has a lot of miscellaneous things related
to loading extensions and the extension system put into a place that does not
depend on Shell or St.
Note that this will break extensions that have with multiple files by replacing
the old uuid-based importer with an object directly on the meta object.
https://bugzilla.gnome.org/show_bug.cgi?id=668429