Do something about loose LuaUI files in default engine installation

[sprunk]

The engine installation comes with a bunch of loose LuaUI files (these https://github.com/beyond-all-reason/spring/tree/BAR105/cont/LuaUI ).

This presents two problems:

• games rely on those files, which is fragile because they are undocumented and just happen to work. But players can just delete or overwrite them etc., it's just a ticking distribution bomb. They should be in some sort of basecontent-level archive for checksum etc.
• many of these files are of very low quality. So just dumping them in an archive would waste a good opportunity to improve them.
• it's also an opportunity to make it more newbie-dev friendly. Apparently the loose files are not enough for an empty mod to recognize a widget placed in ./luaui/widgets out of the box, which is bad.

The first task here is probably to make sure that existing games won't break if we touch these files. Make PRs to games so they have a local copy, or figure out what's safe to modify.