Properties Editor is a bit more picky (compared to the Outliner for
example) when it comes to listening to grease pencil notifiers -- it
requires the action to be set.
So when adding the notifier in the dopesheet from the channels (done in
`achannel_setting_flush_widget_cb`), now add the `NA_EDITED` action.
Maniphest Tasks: T87640
Differential Revision: https://developer.blender.org/D11025
26ab90e49b