Switch from form_alter to menu_alter approach

This module uses a form_alter to take over which menu items are included in the admin screen. However, it does not prevent unneeded items from loading from the initial query in menu_overview_form(). This is because form_alter functions only run after the heavy loading has already happened in the initial form function.

This patch uses a menu_alter instead to prevent menu_overview_form() from ever being called. Instead a modified version, menuadminsplitter_overview_form() is called and it's base query is aware of the depth and menu item limits.

Also, this patch gets rid the use of user objects. I don't see a need for per user defaults. And this module certainly should not do a user_save() in hook_init() on every single page load for admin users.

I'll post the patch shortly.