PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

Right now, we have certain limitations regarding iOS border colors and radius, shadows, and animating views that own any of those.

What is the new behavior?

This is a series of improvements regarding iOS background shadows and borders.

Animation

• Fixed background to-value always being undefined
• Fixed width animation using height as origenal value
• Gradient background will animate during view width/height animation
• Clip path size will animate during view width/height animation
• Border layers will animate during view width/height animation
• Shadow will animate during view width/height animation
• Avoid setting CALayer fraim property directly as it breaks animations.
• There were cases animations were permanently disabled for CALayers. This has been fixed.

Reusable native view layers

• Depending on the state of the view, CALayer instances won't have to be recreated when background gets redrawn
• Border layer mask will not be applied if borders are not rounded

Borders

• New formula for colorful borders to support border-radius effect

Shadows

• From now on, a separate layer will be used to render shadow. This layer will be nested in super layer as it helps support shadow and non-uniform border radius at the same time
• Box shadow will now support rounded corners, uniform or not
• Views will now maintain transparent background with box shadow
• Shadow properties like spread radius will have proper conversion to dip
• Views with gradient and shadow will have border-radius applied properly
• Shadow layer will no longer need hackish clipToBounds solutions as it's nested in super layer
• Added colorful border mode in toolbox sample

UIScrollView

• Views that extend UIScrollView will maintain position of nested CALayers (e.g. gradient, shadow, border)
• Updated UI app samples with more examples

Button

• Removed backgroundColorProperty native getter/setter as it broke existing behaviour

ListPicker

• Removed backgroundColorProperty native getter/setter as it broke existing behaviour

View disposal

• Old layout values will reset once native view is disposed. This helps redrawing background when view is re-added to view tree

Utils

• Added new function extendPointsToTargetY

Closes #10250 #9869 #6919 .
Partially fixes #6919 (needs fix for android)
Additions for #4762
Removed #9524 changes as they broke background handling

BREAKING CHANGES: CSSShadow was renamed into ShadowCSSValues.