Refactor concept rendering to use NSTextView instead of attributed text drawing

[fespinoza]

In order to fix:

• blurry text on external non-retina screens
• drawing efficiency, when dragging concepts on a retina screen

This means the code won't be able to be shared between iOS and macOS by default

this comes from the content of: https://developer.apple.com/videos/play/wwdc2018/221/

in particular:

[fespinoza]

this on its own means a lot of refactoring for the app itself.

another alternative would be to use layers instead, as far as i understand this is the approach that keynote does in order to share code as well between iOS and macOS

a third aspect of this are the refactorings needed to swap approaches