Add an "uninstall" hook

Hopefully another in a series of patches from New module administration page with many new features, this one focuses on adding an uninstall hook to Drupal.

Rationale:
Now that modules are all in their own directories, all non "core" core modules (statistics, etc.) have had their tables encapsulated in .install files which are only executed when they're first enabled. The result is a nice, clean default install with very few tables. However, if you're just playing around to see what features do and then disabling them, the tables currently stay in the DB and clutter it up, along with the variables each module tosses into the variable table.

The solution is to add an "uninstall" hook which removes this stuff from the system when modules are uninstalled. Because dropping tables and variables is a destructive action, we don't want to do this on merely disabling a module (which you might do as a troubleshooting measure, or just to see what effect it has), but rather on an new "uninstall module" action through the UI.

Here's a partial patch which adds the uninstall hooks and drupal_uninstall_module function from the other patch. Missing is a UI and actual "code" to make this happen; not sure if I'll have a chance to work on that before the code freeze, so I would really appreciate someone else picking up the ball on this one. :)

Probably, admin/settings/modules turns into two tabs:

1. administer modules (the current admin/settings/modules screen)
2. uninstall modules

(This is based on Dries's feedback that tabs should be "actions")

We move "uninstall modules" to its own tab, because it's destructive and it would be bad to accidentally click on it.

On the "uninstall modules" tab, it lists any disabled modules and provides an "uninstall" link, which leads to a menu callback which will show a confirmation form, along with what data will be deleted if they proceed (currently, this is just a message like "all books" -- maybe also show a subset of the data?). If the delete/uninstall button is pressed, the module's uninstall hook fires.