fix(core): improve loaded/unloaded handling to be stable and consistent

[NathanWalker]

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.

What is the current behavior?

Various TypeErrors, crashes and unstable behavior would result from inconsistent handling of loaded/unloaded events.
For example, a ListView bound to an Array where the items were programmatically changed which would fire a render of the list would cause undefined errors due to unloaded firing on a TextField tearing down the delegate used inside.

What is the new behavior?

These conditions are now stable and more performant due to unnecessary repetitive setup/teardown.

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✅ Successfully ran 2 targets

• nx run-many --target=test --configuration=ci --projects=core
• nx run webpack5:build

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[triniwiz]

LGTM 💯