[addons] various fixes/improvements

1. Splits "Broken" status away from "Unmet dependencies" as per http://forum.xbmc.org/showthread.php?tid=177482
2. Fixes issue with dependencies being allowed to be uninstalled unless they themselves are broken.
3. Prompts and doesn't allow disabling of add-ons that are dependencies.
4. Reorders prompts on uninstall so user doesn't get unnecessarily prompted with "are you sure" when the add-on in question can't be uninstalled anyway.