Adding a new mouse navigation option to FreeCAD

Introduction
This tutorial attempts to help developers understand how to add their own custom 'mouse' models to FreeCAD. There are currently several options within FreeCAD to customize navigation, they are listed in the Mouse navigation page. Some of the options include: Revit, OpenCascade, Inventor, Touchpad etc...

Prerequisite

 * Familiarity with C++ syntax
 * Ability to compile FreeCAD from source

Relevant source files
At the time of writing this documentation the relevant source code files are located at:
 * Foo is a placeholder for the different mouse/navigation modes, for example: Revit, Inventor, OpenCascade, Touchpad, CAD etc...
 * - Add your new mouse/navigation mode here so it's picked up by the compiler
 * - Add your new mouse/navigation mode here so it's picked up by the compiler

Historical code commits
We can refer to previous code commits that added different mouse modes for orientation. Here are a few:
 * Revit (commit)
 * OpenCascade (commit)