Video groups: add a mark as not played ctx

Description

OP: https://forum.videolan.org/viewtopic.php?f=35&t=157345#p544029

Unique videos have "Mark as played" or "Mark as not played" ctx. Groups only have the "mark all as played" feature.

Expected behavior

"mark all as played" and "mark all as not played" are available

Actual behavior

Only "mark all as played" is available

Screenshot / video

Context

App version

Android version

Device model

App mode

Smartphone

TV

Auto