Add support for opening multiple timelines

[timlehr]

Feature Suggestion

Currently Raven only supports opening one timeline at a time. Adding tabs or multi window support (for the non-browser build) could be really useful to compare multiple timelines without the need to run multiple Raven instances. This would make the software feel better on macOS, where users are used to a document-based workflow in a single application instance.

[yaash45]

I'd like to take a stab at this!

[timlehr]

Initial implementation in #123.