Loadout manager window performance fixes

Changes

Improved the time it takes to open the loadout editor window on a 400x400 map from 5 seconds to pretty much instant
Similarly improved the performance of the generic tab
Re-added the visibility check for greying out items
Other logic should be unchanged

Reasoning

The previous implementation searches through every selectable thing on the map for every item in the list. That's a lot, especially for big and mountanous maps. Now it's only done once each time you open the window or the generic tab. The rest of the lookups go through a distinct list of defs found, which is much, much smaller. That frees up a lot of performance to reimplement the visibility check that was deemed "CPU expensive!" before.

Testing

Check tests you have performed:

[x] Compiles without warnings
[x] Game runs without errors

CombatExtended-Continued / CombatExtended

Loadout manager window performance fixes #3160

Changes

Reasoning

Testing