WIP added on logic for MovieBox/fragment combo trimming

Basic logic is in-place for IsoFileContainerBox.trim_to_time_range() to
 remove the sample references when the time range starts or ends in a
 fragment.

Added IsoMovieFragmentBox.adjust_offsets() and
 IsoTrackFragmentBox.adjust_offsets to rebase the MovieFragment's offset
 based on repositioning of the MovieFragment inside the container.

Added stub implementations of IsoMovieFragmentBox
 .remove_samples_before/after_point() (all DLNA reference content has one
 sync sample per MovieFragment, so not prioritizing this impl).