Restructure the module lifecycle.

Install: <module>_installer::install() is called, any necessary tables
are created.

Activate: <module>_installer::activate() is called.  Module
controllers are routable, helpers are accessible, etc.  The module is
in use.

Deactivate: <module>_installer::deactivate() is called.  Module code
is not accessible or routable.  Module is *not* in use, but its tables
are still around.

Uninstall: <module>_installer::uninstall() is called.  Module is
completely removed from the database.

Admin > Modules will install and activate modules, but will only
deactivate (will NOT uninstall modules).

modules/slideshow/helpers/slideshow_installer.php

@@ -25,8 +25,7 @@ class slideshow_installer {
     }
   }
 
-  static function uninstall() {
-    module::delete("slideshow");
+  static function deactivate() {
     site_status::clear("slideshow_needs_rss");
   }
 }