Add a remove_file_groups command

[mwestphal]

Is your feature request related to a problem? Please describe.
File can be added easily in F3D by dropping them, however there is no way to remove files. Adding a remove_file_groups command would provide such a solution for advanced users. I dont think an interaction is needed for this one.

Describe the solution you'd like

• Add a new command named remove_file_groups and implement it using scene.clear()
• Add a test using a command script