Split up ExtensionUtils module
This is the continuation of !2832 (merged).
Split up the existing ExtensionUtils module into:
- ExtensionManagement: utilities to be shared between gnome-shell, the extensions D-Bus service and the app
- _ExtensionUtilsCommon: shared code for extensions and prefs
- ExtensionUtils: ExtensionUtils version exposed to extensions
- ExtensionUtilsPrefs: ExtensionUtils version exposed to prefs
For now, the different ExtensionUtils modules are almost identical (other than openPrefs() which is only expose to extensions).
This will change in an upcoming MR.