Esc button for escaping full screen and removing markers

[sharoncy]

The Esc button is used to escape full screen, but also to remove markers. This is confusing, especially since I usually work in full screen since the film strip is partially out of view without full screen. Either use a different key to remove markers, or update the ReadtheDoc.

[Tevemadar]

cbef4f5: Delete and Backspace also hide the marker now. Documentation update is still needed.