Smaller border and some more distance between the circles

* Move the Multi-vehicle UI next, not under, the indicators
* Remove the icon

Added Light theme

This is a small experiment into creating a custom theme for QGC.

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.