Layout and stability improvements

• Added support SwiftUI/LNPopupUI popup bar images for crossfade

• Improved LNPopupImageView content mode handling

• Added crossfade between transition target view and popup bar image to better allow for transitions that are not one to one with the bar image, like full popup content transition
• Fixed Mac Catalyst build and also added general Catalyst improvements to the framework

• Expanded the transition system to support more advanced scenarios, such as SwiftUI/LNPopupUI and the LNPopupTransitionView protocol
• Improve documentation and headers with information about transitions
• Deprecated unavailable API has been removed from the header files

• Fixed SPM layout
• Automatically discover LNPopupImageView views inside popup content controllers
• Introduce LNPopupTransitionView protocol
• Rename LNPopupShadowedImageView to LNPopupImageView

• Introduced new convenience LNPopupShadowedImageView class
• Optimized transitions to support LNPopupShadowedImageView instances
  • Generic views are still supported, but using LNPopupShadowedImageView will result in the smoothest transitions
  • Demo project has been updates to use LNPopupShadowedImageView

Improved transition view frame detection

Added support for popup open and close transition:

• The behavior is opt-in
• In your popup content controller, implement the new viewForPopupTransition(from:to:) method to return an image view or a view that contains an image
  • Both standard demo scenes and the demo music app in the example project have been updated to include this functionality

Fix layout issues with SwiftUI image controllers (#597)

Fix a popup bar layout issue introduced in the previous build (#596)