feat: rudimentary markers save and load

[aonmkugs]

Adds a rudimentary way to save markers per video. When you click on the "Record" button the markers are saved in a JSON file.
If such file exists with the same filename (<video_name>_markers.json) in the source video folders it will load it when you load/switch to this video and restore the markers.

The save/load is really a dumb JSON dumping/loading of the GUI variable self.markers.

This create an artifact .json file each time you record a video (as long as there are markers), it might not be what everyone likes so it could be an option or could be decoupled of the Record button, but for my usage I found it useless to make a new button just for this when most of the time if there are markers I want to save them with the source video if I need to generate the project again in the future.