Use formspec version 6 in the main menu

[rubenwardy]

This makes all the tabs in the mainmenu use formspec version 6 and real coordinates. To do this, the local tab has been updated to work with real coordinates

This is a prerequisite to replacing the settings tab with a settings button, part of #13476

To do

This PR is Ready for Review

• Make fields on the left longer, to take up available space minus a margin

How to test

• Open main menu
• Test that the Start Game tab works as expected

[grorp]

I tested this on my Android device with default Minetest scaling settings. Here is a screenshot:

The left side of the tab looks very crowded, although there is still free space at the bottom. The checkboxes even overlap each other: If you zoom in, you can see that they are wider than they are tall. I think more vertical spacing is needed.

This was slightly better with the old version of the tab:

In particular, the checkboxes did not overlap, but were nearly exactly aligned.

Looks good to me otherwise, but I can't look at the code right now. (The "Open Settings" button got even more ugly, but that doesn't matter for obvious reasons.)

[rubenwardy]

I forgot to increase the spacing between the checkboxes to account for the move to real coords, done now

[grorp]

What about this? grorp@4c6f28a

It makes the left side use the full available vertical space again, i.e. the bottom of the last field is at 7.1 - 0.375.

[rubenwardy]

Looks good, applied that

[grorp]

LGTM apart from that.