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 new behavior?

This PR adds flexibility for multiple RootLayouts and the getRootLayoutById function to retrieve one using its view id.
Core will keep a root layout stack just like it does with Frames.
Also, existing getRootLayout function will return the first RootLayout in the view tree.

Additionally:

• Improved error messages
• Fixed a problem that caused a shade cover with gradient not to switch to a colored one
• Added iOS view disposal support to get rid of native gradient layer when needed
• Now, shade cover will calculate its insert index based on existing popup views