multiple crops per marker pair

[exwm]

Please describe the need for the feature:
When marking up a video, I sometimes want to create multiple clips spanning the same time but with different crops. This would be useful for focusing on multiple subjects during the same time of a video.

Please describe the feature and its ideal implementation:

• Switch between multiple, numbered crops when a marker pair editor is open.
• Current crop number out of the total created crops displayed clearly.
• Edit the currently previewed crop.

Please describe any alternatives you have tried or considered:
I currently need to create multiple marker pairs spanning the same times to create different crops. This is inefficient, and does not always work as overlapping markers are not easily selectable with the mouse. Alternatively I can save markers and reload them in a new window to adjust crops. This is a reasonable workaround but again less efficient.