"Remore markers" if there are several in the selection

[vadosnaprimer]

When there are several markers within the selected frames, change the "remove marker" menu to "remove markers", and remove them all if selected. If only removing one is needed, it should be the only one selected.

I forgot about this initially, and when making a movie with tons of markers I realized how much I miss the way to remove all markers in a given segment.

I would do it myself but my plate is currently full.