Holdable modifiers

[Wunarg]

Adds a new modifier which for any holdable objects sets the sprite for its z to -140 aka behind everything except buildings.
This allows weapons and tools to be used in a more hidden way and gives you a greater opportunity to hide things.

For guns and melee weapons this also grants a 0.8 damage multiplier reducing the damage a bit.

It also increase all work costs by 1 difficulty and 20% amount.

[Wunarg]

Now also adds slippery and rubber grip adding new uses for those 2 resources as well as either heavily decreasing or slightly increasing grip force