Refactor files with membership in multiple targets into a Common hierarchy (#352)

* Rough cut at moving HUD views into their own framework.

* Move view code into the HUDViewKit framework directory

* Remove HUDViewKit

* Add GlucoseTrend and NSTimeInterval directly to WatchApp Extension so that it doesn't have to pull in HUDViewKit

* Get all targets to compile with HUDViewKit

1. Make HKUnit extension internal and included it directly into all
   necessary targets to avoid a conflict with the same extension in
   LoopKit

2. Publicize any necessary HUDViewKit vars, funcs

3. Store the Int raw value for GlucoseTrend in WatchContext and create
   a computed property that returns it as a GlucoseTrend object to avoid
   the conflict where both Loop and WatchApp Extension have their own
   separate concept of GlucoseTrend.

* Rename HUDViewKit to LoopUI

* partway through moving layout

* WIP

* Resize stack view to fill container view

* Embed LoopUI framework

* Use NibLoadable for HUDView

* Create a "Common" group for all files with membership in multiple targets.

Author: Bharat Mediratta