clean up things that were broken after renaming `bg_core` to
`bg_terrain`.
also clean up after global rename that added `bg_` prefix to every mod
name.
rename mods; add `bg_` prefix.
add hacky cleanup ABM (couldn't get LBM:s to work), that converts old
nodes into new ones according to this rename.
the cleanup doesn't do anything for items, though.
create mod for player-related stuff and move hand setup from core mod to
this new mod.
add feature where, while moving, players gradually speed up up to a
certain limit. their jump height is also increased while this happens.
