save animations with date,time, in backups folder

whenever the animation editor is closed, save a backup of what you were
working on to "data/animations/backups/previous_session_<date>.txt"
Rename balanim to custom_animation

Caps requested this. Within the meta branch, balanim part still exists
but is deprecated. New features will only be added to the
custom_animation part and the balanim part will eventually be removed.
Please replace your balanims with custom_animations.