Switching venvs without directory tree

[Akuli]

Some people don't like having a directory tree in their editor. The obvious thing to do is disabling the directory_tree plugin. Currently it breaks other plugins (#1445), but it also makes it impossible to switch between venvs. This makes venv related problems extra confusing (#1456).

Maybe there should be another way to choose a venv that doesn't depend on directory_tree?

[Akuli]

user-doc/projects.md will need to be updated when this is implemented.