make a weapon quick-switcher mechanism

[ioxu]

• [x] triangle D-Pad UP button to action
• [x] #45
• [x] #47
• [x] load weapons in and out of the equip slots to the weapon mount
• [x] connect and disconnect fire signals

[ioxu]

(copied from #38) functionally to equip a weapon it needs to be

1. re-parented under player/MeshInstance/weapon_mount
2. signals need to be connected current_weapon.connect("fire", player, "_on_weapon_fire") and weapon.connect("magazine_count_changed", dui_root, "_on_magazine_count_changed")

[ioxu]

not re-hooking up "magazine_count_changed", magazines are not important right now.

[ioxu]

done with #38