VideoPress: consider to add a `Set current frame` as video poster

We could consider adding a `Set current frame` option to the toolbar of the VideoPress video to set the video poster just by clicking on it.

So, the user can position the video in the desired frame and then click on the toolbar button.

Start by locating the VideoPress video toolbar implementation and the existing handling for video posters. Check how the current frame is exposed and how poster updates are stored, then confirm the proposed toolbar option can set the poster to the selected frame and remains available after saving.

Written by the indexing model from the issue text.