Update custom example to latest changes

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

Major update to simplify and document the custom build example

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.