feat: updating all markers

[aonmkugs]

Adds the ability to update ALL the markers in a video at once with the current set of changed parameters.

To use this, you need to hold Shift when pressing the "Add Marker" button ("+"), it will pick the current marker as the reference (or warn the user if there's no marker at the current frame), check the parameters that have changed in the UI in comparison to that marker and will ask the user to confirm if they want to set all these changed parameters to all the other markers.

For example, if the user has changed two parameters and pressed Shift and the "+" button, they are asked to confirm the update of all markers with these values:

The normal workflow to add markers (without Shift) is preserved as it was previously.

Also includes a small syntax cleanup of update_markers and simplification of some code.