rename methods adjusted_* to shifted_*
functional approach
new sub edit_vars()
Intends to provide a hash that can be passed around
for region calculations.
renamed:    RegionMath.p -> RegionComp.p
Clip: provide shifted_* methods
remove comment
remove unnecessary subs
Already works via the same-name methods in parent Track class
fix insert_in_sequence command
Sequences store track names in items attribute
Sequence list() list_output() methods
verify sequence position indices, process sequence constructor args for restore
new command: show_sequence, fix parameters
grammar: verify position indices
check name supplied by user before creating new sequence
new_sequence creates clips
stub in class ClipMark; rename method unadjusted_mark_time() -> time_from_tag()
ClipMark will get its time values from the sequence predecessor
rename Mark method next_sequence -> next_id
new sub: duration_from_tag() for Spacer class
new ::Track method:   is_region()
clarify code:         duration()
new class:            Spacer
Sequence: new method new_spacer()