Allow texture packs to provide fallback button styling via textures

[mruncreative]

Problem

Many form elements like buttons can not be styled by texture packs for games that did not set textures for them in the first place like MTG.
This means texture packs can not restyle buttons for MTG and supporting more games will be harder for texture packs if everyone uses custom styling for everything meaning texture packs will support less games or less texture packs will be created in the first place with those dire prospects.

Solutions

The Minetest engine should check whether texture packs provide textures for things such as buttons and include the file names it checks for in the documentation. 9-slices are already supported. You just need to put in checks and apply them if available. This could also work for the main menu.

Alternatives

live with ugly GUI

Additional context

No response