Allow for remapping just one button in ES

[dfilskov]

Issue description

If you want to just swap two buttons or remap one button you need to remape the entire controller for no obvious reason.

Please make it possible just pick the button(s) you want and remap those.

Btw - for some reason freshly installed Batocera images seem to always lack the es_input.cfg remapping file. When you remape creating that file issues often arise (like long-press / hold not always working correctly). By making it a 'rule' to always make sure that the es_input.cfg is present and correct no these "hidden" surprises should hopefully be discovered and fixed automatically.

[Tovarichtch]

This thread should be merged with #1835 and #1836 because they aim the same feature request: customize ES navigation inputs.