Move cache location methods so they are available in all builds

Add missing header

Add --clear-cache option

Fix typo

Add support for specifying fixed RTK base position

Change to case insensitive compare

Move ArduPilot preflight check handling to generic QGC

Fix mobile builds

Stable desktop version check

Remove unused code

Show the link indicator in the tool bar if two or more links are connected and at least one of them is a high latency link.
Show in the link indicator if a link is inactive.

Make all argument to AirMap queries based on what the screen shows (bounding rectangle and/or radius), respecting the limits imposed by AirMap.

Break apart string of classes into their own files as all in one file became unwieldy
Expose Airspace data in a QML ready format
Split global Airspace from AirMap (Only now I figured out the reason for the pure virtual root classes)
Refactored a bunch of things to make it consistent and/or obvious

Move all airmap related stuff to its own directory

- add multi-vehicle support: each vehicle has now a per-vehicle AirMap
  object that takes care of flight creation, traffic & telemetry
- add a base class API layer so that the interface between QGC and the
  AirMap implementation is clear, and easier to change
- removes polygon upload to vehicle (needs a better solution)

Make AirMapController as part of a Vehicle rather than a creatable QML instance. This is ultimately linked to a vehicle and not the view. Eventually a lot of the code currently in AirMapManager will have to come here as well so the data persists with a vehicle. AirMapManager should only handle the global aspects.
Added a temporary indicator right below the toolbar. It's only visible if a permission (of any kind) exists. I need to understand better how this works before coming up with a more permanent solution.
Restored toolbar to its original state (brand logo and whatnot)

Converted Structure Scan to use CameraCalc such that it now works with
camera specs.

Also hand carried text to speech changes from Stable to master. Plus
unit tests for that.