Switched to faster Array().pop() when possible.

Even if reverse() is called in some cases, I still end up way faster than to
issue a shift() repeatedly, because reordering the indexes takes a lot of time.
Changed prefix from old_ to last_ where needed.

I always use the last_-prefix to remember stuff from a previous loop iteration.
Changed #union html hierarchy to simplify things.

Now I can for instance safely add a button anywhere in the list item, and the
script doesn't look so by stupid cloning it every time.
Moved content-filter items into a filter-item class.
Added 'perform-spatial-operation' look-and-feel.

It does nothing but looks neat. I also see some self-explanatory power.