extensionUtils: Load extensions asynchronously

["Norman L. Smith" <nls1729 gmail com>]

Hello:

I was poking around GS3.7.3 and found what appears to be a bug or 
atleast an issue that has not been addressed yet.
==========================================================
commit 6b40c3974d55c52e67c066da59febc34c6d7c92e
Author: Florian Müllner <fmuellner gnome org>
Date:   Fri Nov 30 18:06:11 2012 +0100

extensionUtils: Load extensions asynchronously
    
Use the new collectFromDatadirsAsync() function for extension
loading as well.
    
https://bugzilla.gnome.org/show_bug.cgi?id=689304
===========================================================

The above commit breaks extensionPrefs/main.js.

When the gnome-shell-extension-prefs is executed with an extension
url or uuid as an argument the extensions are not loaded before 
_extensionAvailable() is called in _onCommandLine in main.js. I did
not see a way to fix it properly in extensionPrefs/main.js.

I created a patch based on the old code which adds a synchronous scan
to the ExtensionFinder class in extensionUtils for use by callers who 
need the extensions populated without delay.

If this issue is not in someone's queue or otherwise is not a problem,
let me know and I will file a bug with the patch.

Regards,

Norman