Track and prefer manual inventory letter assignments over automatic ones.

This fixes the issue of dropping then picking up items removing your chosen key assignment,
as well as sheathing and unsheathing doing the same.

It also avoids automatically assigning items to keys you've manually assigned to something else,
so if you assign your knife to 'k' and put it in a sheath, only another knife of the same type will take 'k'.

I also made it so that the inventory screen colours manually assigned invlets yellow. Not sure whether that's a little too much, but it's useful. You can toggle manual assignment by reassigning the same key.