Support system-wide gschema

[emansom]

When installing the extension system-wide, it currently fails to load producing the following error:

dec 02 21:43:08 x11sslf gnome-shell[2134]: JS ERROR: Extension jiggle@jeffchannell.com: GLib.FileError: Openen van bestand ‘/usr/share/gnome-shell/extensions/jiggle@jeffchannell.com/sc>
                                           settings@/usr/share/gnome-shell/extensions/jiggle@jeffchannell.com/settings.js:15:44
                                           enable@/usr/share/gnome-shell/extensions/jiggle@jeffchannell.com/extension.js:63:30
                                           _callExtensionEnable@resource:///org/gnome/shell/ui/extensionSystem.js:184:32
                                           loadExtension@resource:///org/gnome/shell/ui/extensionSystem.js:385:26
                                           _loadExtensions/<@resource:///org/gnome/shell/ui/extensionSystem.js:633:18
                                           collectFromDatadirs@resource:///org/gnome/shell/misc/fileUtils.js:28:28
                                           _loadExtensions@resource:///org/gnome/shell/ui/extensionSystem.js:612:19
                                           _enableAllExtensions@resource:///org/gnome/shell/ui/extensionSystem.js:639:18
                                           _sessionUpdated@resource:///org/gnome/shell/ui/extensionSystem.js:659:14
                                           init@resource:///org/gnome/shell/ui/extensionSystem.js:57:14
                                           _initializeUI@resource:///org/gnome/shell/ui/main.js:309:22
                                           start@resource:///org/gnome/shell/ui/main.js:186:5
                                           @resource:///org/gnome/shell/ui/init.js:6:17

This PR resolves that issue by adding a fallback to the system-wide gschema if the schemas directory doesn't exist.

[emansom]

If #68 and/or #69 is merged before this: ping me to let me rebase.