session: First notification support

A bit of a rough draft:

About the code/TODOs:

• Two calls to window.present, as it wouldn't work for me otherwise

Other remarkable things:

• Switching to fractal is weird, as you have to first click the notification, you get a second one with "Fractal is ready" and then you have to click that one and then you are switching to the window. If another notification is faster then the behavior is undefined ¯\_(ツ)_/¯

This is just the first part, you can't e.g. control the notification settings

Part of #836 (closed).