extensionSystem: Add "UninstallExtension" DBus method

For those who like their system pure, this provides the ability to purge a
pesky extension and its precious place on your disk space, and in your
"Local Extension" list.

https://bugzilla.gnome.org/show_bug.cgi?id=658612

Conflicts:

	js/ui/extensionSystem.js

js/ui/shellDBus.js

@@ -48,6 +48,10 @@ const GnomeShellIface = {
               { name: 'InstallRemoteExtension',
                 inSignature: 'ss',
                 outSignature: ''
+              },
+              { name: 'UninstallExtension',
+                inSignature: 's',
+                outSignature: 'b'
               }
              ],
     signals: [{ name: 'ExtensionStatusChanged',
@@ -178,6 +182,10 @@ GnomeShell.prototype = {
         ExtensionSystem.installExtensionFromUUID(uuid, version_tag);
     },
 
+    UninstallExtension: function(uuid) {
+        return ExtensionSystem.uninstallExtensionFromUUID(uuid);
+    },
+
     get OverviewActive() {
         return Main.overview.visible;
     },