Item processing stability

[Ziktofel]

What is this fixing or adding?

See #309 and #310

Adds ItemFilterFlags to be processable by pool_filter
Start of splitting the removal of hero items and the related faction items as a preparation for cases where the player will want to play with Nova + Zerg, Kerrigan + Terran, etc
W/A upgrades aren't any longer locked by mission count (they are part of the logic instead)

How was this tested?

Generation and unit tests

If this makes graphical changes, please attach screenshots.

No

[MatthewMarinets]

Lots of changes, I kind of skimmed over most of the rename stuff and that means I didn't go through the removed/excluded stuff in init.py with a fine-toothed comb. Aside from the user option pointing to the "locked" flag rather than "necessary", things look good.

Overall, this seems to cover most of #309 -- the hard part at least. The remainder is the "allow_generation_failures" option, which we can delay comfortably; and various naming/cleanup things like removing origin.