feat: Implementing 'onBackPressed' for frame-root. Useful for meddling with tab-navigated apps

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• 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.
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• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/WritingUnitTests.md.

What is the current behavior?

Currently, 'onBackPressed' callback will never get called in the case of a frame root.

What is the new behavior?

Developers will be able to handle back press for frame root.

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[NathanWalker]

Great contribution @dimitrisrk thank you!

[NathanWalker]

@dimitrisrk published in 6.5.6 now 🤗