This PR is related to #10538, but can be merged independently. Whichever is merged first will cause minor conflicts with the other, but it's a simple rebase that I'd be happy to take care of.

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• 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.
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• 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?

ViewCommon is a subclass of Observable. When overriding addEventListener and removeEventListener, it makes the following changes to the signature:

- addEventListener(eventNames: string, callback: (data: EventData) => void, thisArg?: any, once?: boolean): void;
+ addEventListener(eventNames: string | GestureTypes, callback: (data: EventData) => void, thisArg?: any, once?: boolean): void;

- removeEventListener(eventNames: string, callback?: (data: EventData) => void, thisArg?: any): void;
+ removeEventListener(eventNames: string | GestureTypes, callback: (data: EventData) => void, thisArg?: any);

Although this is allowed in TypeScript, even in strict mode, here are my motivations for removing it:

• there's no real need for it. We can just coerce to string upstream of it. Its one advantage is that it supports expressing multiple events at once, but (aside from there being an appetite to remove support for adding/moving multiple events at once) it seems we're not even using that functionality in practice.
• we're not extensively using the feature anywhere within Core in the first place (only four lines across all of Core).
• it complicates the typings and makes them diverge further from EventTarget.

What is the new behavior?

Accept only strings for event names.

This is not a breaking change within Core (no tests were relying on it, and I've migrated the only four lines relying on it), and any plugins making use of it should see what to do by heeding the updated typings.