Support for MP4 EmptyEditList time-seek insertion

IsoFileBoxContainer now takes a insert_empty_edit parameter which will
 cause it to create an "empty edit" entry in the EditListBox with a duration
 equal to the requested start time and adjust the movie/track durations
 to account for the edit.

Not printing decimal seconds in all to_string()s that include duration.

Enabled selectively enabling the empty edit via the command line test program.

Tested with various time ranges on
 bbb_avc-main_720x576i50_2M_heaac51_48kHz_160kbps.1_1.mp4