Feature Request: add actions when running

[elig0n]

Suggestion Explanation: Add custom actions (1..9 and default) while feh is already running.

Possible Use Case: When sorting (moving) images into various directories this can allow coming up with desired move locations (or more generally actions) while sifting not requiring to lose position and restart feh...

Possible implementation: via the menu system, and/or a key open an input dialog for an action number + a command.