self-document and un-hardcode the prone sequence prefix
remove unused duplicate stand sequences
remove bogus tab
fix infantry without idle anim staying in fire animation state

closes #5884
add missing TakeCover trait
replace bogus tab with space
remove reference to non-existing idle sequences
Merge pull request #6331 from Mailaender/infantry-noidle-fix

Fixed Dune 2000 infantry not showing the waiting animation when ordered to stop firing