Adds a filename split seperator setting that lets you choose between old style `_` and `.` because dot is the only char that isn´t allowed in playername but in texturenames.
Default value is `_` to keep it compatible with older versions, while `.` offers a solution to fix #54.
i.e. the formspec v4 rewrite
Requires UI "version-2" tag or commit a7556c50 or later and
and Minetest v5.4.0 or later
This also makes it work properly in UI's "lite" mode.
Check for core feature `httpfetch_binary_data` to run the updater properly.
This requires Minetest cb00632 or newer.
README.md: Add documentation for the updater
