extension-prefs: Give the UI a bit of GNOME 3 treatment

The extension-prefs UI has never been great, but as the GNOME 3
design patterns are evolving, it is starting to look seriously
outdated. Modernize the UI a bit to have it fit in a bit better.

https://bugzilla.gnome.org/show_bug.cgi?id=730829
extensionPrefs: Add switches to enable/disable extensions

Bring the extension-prefs tool in line with the mockup by adding
switches to enable/disable extensions, similar to the extension
page in gnome-tweak-tool.

https://bugzilla.gnome.org/show_bug.cgi?id=730829
extensionPrefs: Skip main window when launched with a UUID

The extension-prefs tool is used by gnome-tweak-tool and the
extensions web site to display preferences. However as those
already implement their own extension lists, the main window
is not useful in that context (to not say it is rather silly).

Just skip the main window and only show the specified extension's
preference dialog in those cases.

https://bugzilla.gnome.org/show_bug.cgi?id=730829