Add "later" option on shutdown via Power / Restart screen

[prl]

Powering off (to Shutdown) via the Power / Restart screen (default long-POWER) while there's a recording in progress brings up a popup, but its only options are to shut down immediately, or to cancel the shutdown. There is no opportunity to specify to go to standby and then shutdown when recordings and/or tasks have completed.

This reflects the capability in the DP series shutdown, and offers the choice of the standby-then-shutdown action taken when shutdown is direct via a POWER button press (e.g. when MENU>Setup>System>Button setup>Action on short POWER is set to Immediate shutdown.

This clarifies and replaces Proposal #48 No "Later" option for Shutdown when a recording is active, which has been marked as "duplicate".

[prl]

Issue #48 was marked as a duplicate of this issue.