FreeCAD GUI Widgets toolkit

Background
Design456 is an external workbench that aims to provide a Direct Modeling solution within FreeCAD.

FreeCAD GUI Widgets toolkit
In the effort of making new tools for direct modeling, a graphical user interface (GUI) is needed. A widget system is required to let the user interact with the 3D view. Often FreeCAD implemented this interactive part as a ViewProvider. These ViewProviders are made for each tool and are specific for each tools. This doesn't serve Design456's agenda as it would make producing new tools difficult. That is why the necessity to figure out how Coin3D works, in other words how drawings directly to the scenegraph is implemented.

This GUI toolkit will in the beginning try to produce different kind of primitive drawings which then can be adapted by different widgets (i.e square, circle, triangle, ellipse etc...) For example an arrow that a user can push/pull to get the effect of extruding or pushing a face of a 3D object.

As I am inspired totally by the simplicity of FLTK GUI Toolkit used for making Windows/macOS/Linux GUI interfaces in cplusplus and other languages. The Fr Widget toolkit will be inspired by FLTK so understanding the toolkit shouldn't present much difficulty.

Installing the GUI toolkit
The Design456 Workbench is a prerequisite to be able to use this widget system. There is an option to specifically import the toolkit via python code, but Design456 Workbench is still required