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, even if a view is unloaded, pseudo-class listeners will not be disposed.
At the same time, once view is loaded again, pseudo-class handlers will add new listeners resulting in a leak.
There's also another case of iOS button which can keep reference of their previous native view if removed and re-added to the view tree. That is also a defect caused by missing pseudo-class handlers cleanup.

What is the new behavior?

This PR ensures that pseudo-class handlers will follow the subscription cycle of adding and removing listeners.