Android: Allow players to change the button layout

[AdityaMH]

Problem

As you can see, jump and sneak button blocking my shortcut inventory

Solutions

Add custom layout button only for touchscreen device.

Alternatives

Actually no other alternative

Additional context

No response

[grorp]

Changing the button layout currently requires forking Minetest. A number of forks have been created for this purpose, including ROllertest, the NodeCore Android app and partially MultiCraft.

To make the official app viable for more players and to avoid mobile players having to maintain their own Minetest forks, we should allow players to customize the button layout.

I support this 👍

I'm working on this, but it will be some time until I'm done. You can look for the branch in my fork if you really want to, but the code is a mess.

I'm prioritizing this over the "add on-screen dig and place buttons" part of #13229 because having more buttons will make the problems with the current button layout worse, and hardcoding a different one isn't really a solution.

We should also allow game/mod makers to select a set of buttons to be shown and to provide a default layout and button textures. However, player customization is more important and can be done first IMO.