"User Items" on HUD (AHI) should be movable

The option to display "User Items" on the HUD is very helpful. It would be great if the user could select the location of the items. As it is, they interfere with other data on the screen.
It would also be great if some of the fixed items on the HUD could be removed. For example, the time, the rssi, or the AS for users that don't have an AS sensor.

Start by locating the Mission Planner HUD implementation and the code handling the "User Items" display. Trace how HUD elements are positioned and which fixed items are always shown. Done means users can choose the location of User Items and remove the listed fixed items, including time, RSSI, and AS.

Written by the indexing model from the issue text.