Add icon path to FreeCAD resources list

[hasecilu]

Instead of hard-coding the full path to the icon we can add the directory where icons are saved to the FreeCAD resources list with FreeCADGui.addIconPath(), so when assigning the icons to the commands there is no need to provide full path.

I noticed that your ViewProvider is still working with no changes on my computer, could you test it works after saving a file?

When I did this change on STEMFIE WB (check this commit) I needed to change the View Provider but I'm not using any dumps, loads, __getstate__, __setstate__ methods (I don't have JSON serializable errors, or at least nobody has told me), in other hand attach() method just caused me errors so I deleted it.

[looooo]

Not sure if this is a good idea thinking about name-clashes. I guess if we choose to go this way we should prefix the icons with something like freecad_gears. Correct me if I am wrong.

[hasecilu]

I have seen some WB uses same name as the command and since the command is prefixed with WB name would not be clashes. I'll make that change.

[looooo]

What is the general advantage of doing it that way?

[hasecilu]

• It would be available for other users to use, in my case I'm creating a gear command on STEMFIE WB where I simply add a STEMFIE-head hole, I would be able to reuse the same icon without having to copy the file on my repo.(commit)

• In other hand, other addons that manipulate the UI of FreeCAD such as https://github.com/APEbbers/FreeCAD-Ribbon manipulate the resources provided by the WB and to have the command icon the "pixmap" variable is read and it's cleaner to have only the name of the icon than having the full path, check this comment: Bug: Replace hardcoded icon paths with current paths APEbbers/FreeCAD-Ribbon#93 (comment)

[hasecilu]

Icons working: