Refactor/fixes for ProximityRadar and friends

- add NOTIFY for distanceSensor property
- add DistanceSensorReadingsChange() callback prototype

- revert excessive NOTIFY
- adding MaxSensor into DistanceSensor frame handling

- add proximity radar view in map view
- add proximity radar view in video view

change distance label positioning in video view

clean up unwanted comments

- space up proximity sectors
- revert unwanted changes in qgroundcontrol.qrc

squashed commits

Unified photo/video control which works with mavlink cameras and just video stream

Telemetry Values bar at center bottom

ToolStrip rearchitecture for encapsulation and customization

Rework Fly View into separate layers which can be customized at each layer.

More work on encapsulation in Fly View

Fly View: Move business logic inside controls

Mavlink terrain protocol support

Move PreflightCheckList to custom builds only. The model, which used to be implemented within BuiltinPreFlightCheckModel is now within the control itself. It is up to the implementer to define a proper model.

* QGCCheckListItem -> PreFlightCheckButton
* CheckList -> PreFlightCheckList

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

Mostly accomplished by delay loading Qml elements