add some foldable vehicle parts & make some parts foldable
make simple wheelchair vehicle

it's non-functional as it does not have pedals, which would be silly on a wheelchair, but we don't have hand-pedals.
add generic folded vehicle item type
Use name, description, volume and weight from item::item_vars if found
Adds generic foldable vehicles.
Add hand rims, work similar to pedals

As with pedals, hand rims can not be used together with engines. Hand rims and pedals also don't work together.
remove pointless clearing of the vector

as tested the map does not contain the entry, so it's per definition already empty
Add wheel chair wheel item, wheel chair vehicle part, hand rims item

Adds recipes for them, too.
Save and load has_hand_rims status
Merge pull request #7078 from BevapDin/folding-vehicles

Folding vehicles