Work on multiple Opossum files simultaneously

[mstykow]

Is your feature request related to a problem? Please describe.
As a user, I'd like to work on multiple different Opossum files at the same time.

Describe the solution you'd like
Main thread should be able to spawn multiple independent renderer threads.
It should also be possible to close individual windows without closing the entire app.

Additional context
See here for example: https://spin.atomicobject.com/multiple-windows-electron-app/