cs_panel.py: Error handling for panels on unavailable monitors

[MellowKyler]

If a panel definition exists for an inactive monitor, launching the panels settings menu will fail.

In id_or_monitor_position_used we check if a position is used by going through monitor_layout[monitor_id]. This can result in an index out of range error if the length of monitor_layout is not greater than monitor_id. See #12378 and #12397.

To reproduce:

1. Go to the display settings menu and disable a monitor with a panel (or simply unplug a monitor with a panel)
2. Attempt to launch the panels settings menu

This implementation does not remove the panel definition and it will still be available when the monitor is re-attached.