browser-plugin: Provide new APIs for launching extension preferences

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

js/misc/extensionUtils.js

@@ -99,6 +99,7 @@ function loadMetadata(uuid, dir, type) {
     meta.dir = dir;
     meta.path = dir.get_path();
     meta.error = '';
+    meta.hasPrefs = dir.get_child('prefs.js').query_exists(null);
 
     return meta;
 }