Added `merge_progressive_render_sequence` command

Added the `merge_progressive_render_sequence` for sample-merging sequences of EXR files. The
already-existing `merge_progressive_renders` command only performed on one frame at a time.

This commit also removes the last traces of the preview generation that
half-happened while merging. It was intrinsincly flawed and was superseded
by the `exr_sequence_to_jpeg` command.