Add hud element hotbar partitioning

[cx384]

• Add a feature to the hotbar HUD element, to specify which parts of the hotbar_source it uses.
• With this you can split your hotbar into multiple parts and display them, e.g. in two columns. So it is more flexible than using the hud_hotbar_max_width setting.
• Successor of Replace hotbar_itemcount by hotbar_source #15905

I decided to use the world_pos parameter of the HUD element def, since it consists of three numbers, which is exactly what we need, a hotbar doesn't relate to a world position anyway, and the other possible fields (number, item, scale) can be used for something else in the future, e.g. if we add multiple hotbar sources for different purposes, e.g. a second hotbar which is only active/changes the selected item when you hold/press a specific key. If world_pos is too confusing, an alias inventory_pos which just overlaps with world_pos could be added.
However, feel free to complain if you would like it to work differently.

Screenshot (click me)

To do

Ready for Review.

• Waiting on Replace hotbar_itemcount by hotbar_source #15905

How to test

• Read the lua_api.md changes regarding the hotbar element
• Start devtest
• Execute /hudhotbars three 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.