Drop JointJS+ dependency

[alexandernst]

The project can't be executed locally unless the user has a JointJS+ license, which is a limiting factor both for people who want to contribute to us and for people who would like to run the project on their own machine.

We depend on the following JointJS+ packages:

• joint-clipboard
• joint-command-manager
• joint-format-raster
• joint-format-svg
• joint-free-transform
• joint-keyboard
• joint-navigator
• joint-paper-scroller
• joint-selection

Ideally we'd reimplement all of these packages and drop the dependency entirely, but the bare minimum for allowing users to run this project locally is the joint-paper-scroller package. By implementing that package we'd be able to run the project with a limited set of functionalities, but it would still be better than not being able to run it at all.