Core plugin: navigation

[rigor789]

Implement the navigation plugin:

• $navigateTo(target: Component, options: NavigateToOptions)
• $navigateBack

Ideas:

• Add a key to NavigateToOptions and allow passing that same key to $navigateBack to navigate back to a specific page in the backstack - identified by that key. Use case: User can navigate to different pages and back, and on any of these pages, a button can navigate back to the n-th page without clearing history.

Things to keep in mind:

• $navigateTo needs to attach event listeners to the Page - to clean up after the page is no longer needed
• $navigateTo should accept a frame id - and resolve it to a Frame instance. getFrameById will only resolve frames that have already navigated - so we need to maintain a map of ids and frames, so we can resolve them.
• $navigateTo - re-think the resolveOnEvent option from 2.x - I think it makes more sense to do that in userland?
• $navigateTo - (TBD) BREAKING CHANGE (compared to 2.x): default frame will be topmost instead of default. Pros: less verbose to navigate last frame Cons: can be "random" and navigate the wrong frame in some circumstances.