Refactor the HUD and the killing mechanics

[nicemicro]

Currently, only the infiltrator kill and reload actions have a HUD, but it is limited to a gun that only shots once, and it reacts independent of whether the killing is enabled or not.
To close this issue, one must implement:

• A universal HUD for all players
• Instead of using the role of the player to simply show a gun, there should be a table distributing locked items (currently only the gun, but later a poison injection, etc. might be added) to certain roles, and the HUD should show the locked item(s) the player got assigned.
• Active "gun" icon only when there is someone in range to kill
• Items: the item picked up should appear on screen (and it hides the gun icon). It should allow the users to drop or use the item.
• Dynamic "Use" icon for interacting with tasks.

EDIT
A PR addressing this issue should also resolve the fact that item pick up and task interaction uses the same collision layer but uses totally different mechanisms for the interaction.