Major update to simplify and document the custom build example

Use parsebin for automatic stream demuxing and parsing; Automatically insert rtpjitterbuffer for RTP streams to avoid video stuttering

qobject_cast is times faster then dynamic_cast for objects
that inherit QObject. also, use auto keyword to not repeat
the class name

qobject_cast is times faster then dynamic_cast for objects that
inherit QObject. Also, use auto keyword to not duplicate the
name of the Class

Ligther Button, Primary and Hover + New alert color

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.