refactor(tabs): strip item appearance and creation

[vchimev]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/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/DevelopmentWorkflow.md#running-unit-tests.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/WritingUnitTests.md.

This pull request:

• implements strip item appearance for iOS
• refactors strip item creation for both platforms
• fixes a crash on swiping a content item for iOS
• adds tab strip items test pages

Implements #7436 and NativeScript/nativescript-angular#1884.
Fixes #7459.