Themes need an installation status

Problem

1. Theme settings/configuration/variables are never removed.
2. All themes are loaded unconditionally, regardless of whether they are going to be used.
3. Base themes are initialized and used even if they are disabled, and even if they are not installed at all.
4. Themes can have configuration and can participate in hooks, but they are not able to maintain their data in any way.
5. String translations of a theme are re-imported whenever a theme is re-enabled.
6. The update system/manager is not able to determine whether a theme is actually used.
7. ...
8. Themes cannot be installed or uninstalled.

Goal

• Sanity.

Proposed solution

1. Treat themes like modules. A theme is either installed and enabled or it is not. An extension that is not enabled cannot be used.

   Essential impact:

   • Admin themes have to be enabled in order to be used.
     #542828: Do not special case disabled admin theme
   • Base themes have to be enabled in order to be used by sub-themes.
     #1663142: Require that the base theme be enabled to use a subtheme
2. No change: Themes can be enabled and disabled.
3. Separate follow-up: Allow to uninstall themes.

Particular @todos to resolve/revert

• #545518: Move Locale module specific code out of module.inc and system.module