Add keybind to swap items between hands

[LizzyFleckenstein03]

Adds a key binding ('F' by default) that swaps the item currently in your main hand with the one currently in your offhand,

To do

Depends on #11016 to be merged first.
Otherwise ready for review.

In the future, this should be added to android UI as well.

How to test

1. Follow testing instructions for Dual Wielding #11016
2. Press F while there is an item in main hand and offhand is empty, verify it works
3. Press F while there is an item in offhand and main hand is empty, verify it works
4. Press F while there is an item each hand, verify it works
5. Press F to pay respects, verify it works

[erlehmann]

F