Simultaneous Display of Mission, Geofence, and Rally Points in PLAN Screen

I would like to simultaneously display and edit missions, geofences, and rally points in PLAN Screen.
This would enable users to view and edit missions while also creating geofences and rally points in the context of the mission they are planning.

As it currently stands, users must toggle between "MISSION", "FENCE", and "RALLY" options via a dropdown menu located in the upper right corner of the PLAN Screen. This functionality limits the display to only one selected item at a time.

Proposal:
- Maintain the current dropdown menu for selecting the primary editable item ("MISSION", "FENCE", or "RALLY").
- The selected item from the dropdown menu would be displayed in an editable mode, allowing users to add, remove, or modify points as currently possible.
- Items not selected in the dropdown menu would be displayed in a read-only mode. This would allow users to see all elements (mission waypoints, geofence boundaries, and rally points) overlaid on the map simultaneously, but only edit the item type they have selected from the dropdown menu.

By the way, it's noted that previously, the right-click menu in the PLAN screen allowed for simultaneous creation of missions, geofences, and rally points—a feature no longer available when a vehicle is connected, as these options are removed from the menu.
https://github.com/ArduPilot/MissionPlanner/blob/latest/GCSViews/FlightPlanner.cs#L2661-L2673

Start with the linked FlightPlanner.cs section around lines 2661-2673 and trace the PLAN screen's dropdown and right-click menu handling. Determine how mission, geofence, and rally elements are displayed and edited, then verify that all three can be overlaid while only the selected type remains editable.

Written by the indexing model from the issue text.