Allow any Route to be offstage for an undetermined period of time

[rrousselGit]

Note: I plan on implementing this feature myself, but it will likely require a breaking change, and the change is not trivial.

Use case

A package I'm working on needs to act as a man in the middle between a Navigator.push and the appearance of the route on the screen.

The idea is, a NavigatorObserver may force pushed routes to be in an offstage state similarly to how Hero works. But that offstage status may last for a lot longer than one frame. In the meantime, the previous route should stay visible and interactive.

Currently, doing so leads to a black screen.

Proposal

• move offstage property from ModalRoute to Route.
• offstage routes do not make the previous route invisible even if they are opaque.
• offstage routes don't capture clicks.
• going from offstage to onstage must not destroy the Route state.
• a route that was never rendered as onstage before should correctly start its transition animation from 0 when it appears
• routes may be popped when offstage

Misc:

• HeroController may mess up with the opaque variable.
• We'll probably want to move the "animation should start only when the route becomes onstage" out of HeroController and into Route itself.