extensionSystem: Add a method to unload an extension

https://bugzilla.gnome.org/show_bug.cgi?id=677586

js/ui/extensionSystem.js

@@ -167,6 +167,23 @@ function loadExtension(extension) {
     _signals.emit('extension-state-changed', extension);
 }
 
+function unloadExtension(uuid) {
+    let extension = ExtensionUtils.extensions[uuid];
+    if (!extension)
+        return false;
+
+    // Try to disable it -- if it's ERROR'd, we can't guarantee that,
+    // but it will be removed on next reboot, and hopefully nothing
+    // broke too much.
+    disableExtension(uuid);
+
+    extension.state = ExtensionState.UNINSTALLED;
+    _signals.emit('extension-state-changed', extension);
+
+    delete ExtensionUtils.extensions[uuid];
+    return true;
+}
+
 function initExtension(uuid) {
     let extension = ExtensionUtils.extensions[uuid];
     let dir = extension.dir;