Make most requirements optionals

[inkhey]

Make possible to install only some requirements, leading to support only some kind of preview allow us to:

• Avoid install unneeded dependencies
• Make support for some platform more easily (if 3d preview cannot work on python 3.9, it should not block us to make jpeg or video previews).
• Help project based on preview-generator to not requirements these dependencies, allowing for example a "minimal" version of tracim without inkscape, libreoffice and vtk.

[inkhey]

available in preview_generator 0.22