Custom Gun Continued

• Add Designated Marksman Rifle (DMR) to gun category
• Add creative tab items using resource location customguntab:{tab_category}_icon (e.g. customguntab:ar_icon)
• Add hide_in_game to gun index, ammo index

Fix:

• Fixed horizontal mirroring when items are rendered as inventory slot textures in item frames

For Developer:

• Recompile any code that uses GunCategory