Add right-click context menu to tree

[andersmmg]

Is your feature request related to a problem? Please describe.
It can be a bit of a pain to keep going up to find the right button to add and remove entities, etc.

Describe the solution you'd like
Add a right-click context menu to items in the tree, with options for the buttons like add entity, delete, generate new ID, and anything else that makes sense

Describe alternatives you've considered
Currently you have to select them and go up to find the correct icon each time.

Additional context
None