category_resave needs to use Batch API, and improve usability

The new category_resave module is now an important part of 5.x->6.x upgrade path. If someone previously used category_display module for TOC trees (and the like), but not category_menu (I've disabled that on my site for years, due to it's serious bugs on 5.x and below), and they want to keep the behavior after upgrade, they need to enable new category_menu, and rebuild the structure using category_resave. (BTW, this might be mentioned somewhere, perhaps UPGRADE.txt is a good place?)

Unfortunately, the current state of category_resave is feeling a bit unfinished, and nowhere near usable, due to the following two problems:

- The UI is very confusing. There's no mention anywhere, about how to use this module, and when enabled, it doesn't show up on any of the expected places like admin/settings/category_resave, admin/content/category/resave, operations on node administration screen or the like. I finally had to read through the code, to figure out where to look for the functionality. Really, having to enable a block on the blocks administration page, and then performing administrative tasks from a sidebar is *very* unusual. So my proposal here is: Turn the form into a normal local task (tab) on the category administration page. (It also means simplification of the code - hook_block() implementation not needed anymore.)

- The module doesn't work for large sites, due to php timeout. For example, my site have about 1000 nodes assigned under some containers, but category_resave only saves <10 nodes before timing out. It definitely needs to use the Batch API. I implemented this, so now the function category_resave_container() only just collects a list of nodes into a batch, which is then run through the Batch API (takes half a hour on my site!).

I also cleaned up some obvious cruft - there seems to be leftovers from something like redirection to a dedicated processing page (category_resave/###), which is not used (all the processing was called directly from the form submit handler, already), and doesn't work anyway (the container id given as argument not processed at all). Also the $form['#action'] and drupal_get_destination() stuff did quite nothing. Since the Batch API integrates nicely into a form submit handler, and it was only accessible through a form submit already, I left it at that, and removed the other cruft.

Then, I see no point in the additional check for nodes (assigned content) in category_resave_container(): If we already selected only nodes assigned to the given category (with the query), why collect categories for each of these nodes again, and check whether the first of them is what we're processing? If the node was triggered for re-save by more assigned categories, the check for already processed nodes will catch that anyway.

Attaching a patch, and since it's bigger than the actual module, attaching also the .module file for reference and easier review.