IITC Plugins Selection implemented

Hi,

I've added new menu item (Select Plugins) and a AlertDialog selection of which user scripts the mobile app should load.

![Screenshot_2013-03-19-18-02-10](https://f.cloud.github.com/assets/995948/278371/365ddbc8-90e4-11e2-86c2-89eb4cd0b5b4.png)
![Screenshot_2013-03-19-18-13-24](https://f.cloud.github.com/assets/995948/278372/36a1c734-90e4-11e2-8427-81a8deeb0599.png)

The selection is persisted as preference (one per xxx.user.js, generically, so that changes to assets folder don't influence the compatibility).


Thanks for your contribution!
Saran