[DRAFT] Fix jumpy animation with SwiftUI

[sestevens]

Summary of changes

• Fix jumpy animation that occurs when presenting Drop-in from a SwiftUI view (reported in SwiftUI layout is still broken on a presentation sheet.  #266)
• Add a (very simple) SwiftUI demo to the demo app
• Add a "UI Framework" option to the demo settings to allow toggling between the UIKit and SwiftUI demos

From what I can tell, the jumpy animation is the result of adjusting the top constraint on the content view (which contains the payment selection sheet) in BTDropInController's viewDidAppear method. To fix this, I removed the top constraint and set a height constraint instead. Previously, we were animating the top and bottom constraints to make the payment selection sheet slide up from the bottom of the screen. Instead, we can keep the height constraint constant during the animation and animate the bottom constraint only.

To do

• Fix spinner sliding in from the top left when the payment selection sheet appears (it should just be in the middle of the sheet)
• Fix layout in SwiftUI demo (for some reason the Drop-in is not extending to the bottom of the screen)

Checklist

• Added a changelog entry

Authors

List GitHub usernames for everyone who contributed to this pull request.

• @sestevens

[jaxdesmarais]

Closing this PR since we haven't touches it in 3 years