Feature Request: Custom HUD

#### Issue details
There are a lot of new features being released every day and the HUD is one of the best tools for monitoring live data. That being said, I think it could improve with a few options.

- Follow the dashware concept of custom graphic data streams at a user selected locations. Allow users to code and contribute their own graphics.
- Allow basic equations to manipulate/combine data into a custom variable that don't exist on mavlink.

#### Version
All

#### Platform
[ x ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
All

#### Hardware type
All

#### Logs
N/A

The issue concerns Mission Planner's HUD but names no files, tests, or entry points. Start by locating the HUD implementation and clarifying the scope of user-defined graphics, placement, and equations. Done would require an agreed design and working support for the requested custom data displays and calculated variables.

Written by the indexing model from the issue text.