[TF2] Add priority item system for stock items in Armory

[bobatealee]

This pull request updates the Armory (Mann Co. Catalog) in the following ways:

• Added a 'priority items' system, meant for stock items
  • Priority items are defined manually and independent of the item schema, using schema indexes (e.g. 199 = Shotgun)
  • Priority items get moved to the front of the list of items in the Armory, ignoring their positions in the item schema and respecting filters
  • This allows stock items to be grouped together and mitigates various oddities caused by schema order (Sapper and Construction PDA being isolated on Page 34)
  • Items that only exist in Normal quality (Destruction PDA, Disguise Kit) have the Unique quality forced via a fake Armory entry (mirrors "Show Stock Items" checkbox in backpack)

Before	After

• Allowed Building, PDA, and PDA2 slot weapons to show in the Weapons filter
  • This allows Sappers, Engineer PDAs, Disguise Kit, and Spy watches to be considered weapons by the filter

For Destruction PDA and Disguise Kit to show in the Armory, show_in_armory must be applied to their entries in the item schema. I've created a ready-to-use snippet for this, available here (original for comparison).