Shop Cart v1.0.0.0

Shop Cart adds a proper shopping cart to the in-game shop, so you don't
have to buy vehicles, tools, and pallets one at a time.

Add as many different items as you want to the cart, set a quantity for
each, and see the combined total before you commit - then buy everything
in one go with a single click.

Features:
- "Add to cart" and "Cart (n)" buttons right on the buy screen for
vehicles, tools, and pallets
- Adjust quantities or remove items straight from the cart
- Clear the whole cart in one click, with a confirmation so you don't
wipe it by accident
- "Buy all" purchases whatever you can afford and leaves the rest in
the cart instead of blocking the whole purchase if you're short on
cash
- Works in multiplayer

Not currently supported: buildings and other placeables. Since those
need to be positioned in the world rather than just spawning near the
dealer, they still use the game's normal one-at-a-time buy flow for now.

Feedback and bug reports welcome!