Adds a lightweight comparison tool to the vehicle and equipment shop.

Open an item in the shop configuration screen, press the Compare button or J,
and the mod will add the current item and selected configuration to a temporary
comparison list. Up to five items can be compared at once.

Opening a card returns to that item with its saved configuration selected.
Compare controls are automatically disabled in workshop and vehicle contexts.

The comparison covers common shop data where available: price, power, speed,
working width, capacity, required power, weight, upkeep and simple value metrics.

Changelog 1.0.3.0:
- Restores the saved configuration when opening an item from comparison.
- Disables comparison controls outside valid new-item shop contexts.