Fix MP5K using incorrect NPC activities

[Blixibon]

The MP5K currently uses ACT_RANGE_ATTACK_AR2 as its attack activity, while using SMG1 activities for everything else. This causes the hand position of NPCs to snap to the AR2 position, which isn't correct and looks unnatural. This PR changes it to use ACT_RANGE_ATTACK_SMG1 instead.

However, the SMG2 worldmodel's animations are designed to use ACT_RANGE_ATTACK_AR2, so in order to fix that without recompiling the model or breaking workshop addons, a hack is included for the worldmodel to fall back to ACT_RANGE_ATTACK_AR2 if ACT_RANGE_ATTACK_SMG1 isn't available.

This PR also fixes the SMG2 using pistol holster/unholster animations.

[1upD]

I'm a little bit surprised by this. Do they really use an attack animation that doesn't line up with the model? Does the SMG1 animation look better for this? I think you've already validated this so approved