Decouple UI from Core - i.e. all UI items into separate core package to simplify maintenance

[NathanaelA]

Decouple the UI from core. Potentially something instally like @nativescript/core-mobile-ui

Pros:

• I believe this would allow more people to easily maintain different UI elements. In addition people could volunteer to be a maintainer of a single UI component so say I said I wanted to maintain the ListView, then that would be my position to merge PR, and fix issues related to just the ListView
• Each should be a package in probably its own mono-repo.
• Each component would have its own demo and tests.
• Allows users to easily work on a single component without being overwhelmed.
• Allows depreciation of UI components that are better handled in the community.
• Allows easier ability to add additional UI's (like Windows, Mac, Fuschia, Web) as then they would be all in a single location, however the build screen could create a @nativescript-core-web-ui, @nativescipt-core-node-ui and @nativescript-core-mobile-ui depending so that the end user could install which ever UI's he is building hi app for.

Cons:

• More repos....