Improvements on editor based on old generator

[LucasWillemsens]

While finalizing our work on the old generator, some remaining problems came up:

1. Edges: editing does not work properly. This is noticeable when the current type is not loaded and a new one cannot be selected. There also is no save button while editing. Also, deletion of an edge might not result in deletion of it in the metadata. Dependency would be nice to have as a relation type in class diagram too. Relationships with guards or important types like Includes and extends should have this info visible in diagram.
2. Paths of edges is suboptimal: In use case diagrams, when use cases are close to each other, the relationships can bug out. Also, unnecessary overlap between relationships is not accounted for. related issue: studio #5
3. Enums cannot be selected as an attribute type of a class.
4. Copy / Paste does not work, which makes creating diagrams with many similar nodes a chore.
5. Merge / Decision nodes can both have multiple incoming and outgoing edges at the moment, while this goes against their specification. Ideally, if one were to add an extra incoming edge to a decision node, a merge node would be created automatically that is linked to the decision node instead
6. New project on homescreen does not always work
7. An undo feature would be nice
8. Default values when creating new edges and nodes would help solve a similar problem as 4. (missing copy / paste feature)