Replace hotbar_itemcount by hotbar_source

[cx384]

• Resolves Allow for different inventories to be show in the hotbar #11723 and resolves Add support for combining multiple inventory lists into the hotbar #8297
• Adds a hotbar_source and thus dehardcodes the "main" player inventory.
• Decouples the hotbar HUD element from its source and the underlining player wielded item mechanics.
• Adds features to the hotbar HUD element, to specify which parts of the hotbar_source it uses. Moved to a followup PR.
• Should be backwards compatible, get_wield_list() already existed, and I added a note to not use set_hotbar_source() together with outdated mods that rely on the list being "main".

To do

Ready for Review.

How to test

• Read the lua_api.md changes
• Start devtest
• Execute /hudhotbars three two times and notice that the hotbars behave like specified in lua_api.md
• Change your window size and test if the hud_hotbar_max_width setting still works as expected.
• Maybe test changing the wield item by pressing number keys.
• Execute /hudinventories and see that it still behaves the same
• Execute /unittests

[cx384]

I removed the hotbar element feature and will put it in another PR.
Maybe it will make this PR more likely to be reviewed.

(We will probably still want the feature, though, and I had to implement the functionality anyway to handle the hud_hotbar_max_width setting, and it was helpfull during testing.)

So with this PR you can no longer change which parts of the hotbar the hud element uses, it always uses all inventories of the hotbar source. (Like it is the case in master, but only with a fixed inventory.)

[SmallJoker]

I like the concept of this PR, thus here's the first iteration of my review.

[cx384]

Thanks for the review. I fixed most things, I hope.

[cx384]

The macos pipeline seems to be broken, for some reason, but this PR is ready for review again.

[cx384]

Rebased.
I consider this a step towards dual wielding, so don't hold back on reviewing this.

[cx384]

Rebased (and some trivial code style fixes)