These methods are useful for verification in tests, e.g.
```js
expect(await page.isEnabled(':text("Remove All")')).toBe(false);
await page.click(':text("Add Item")');
expect(await page.isVisible('.item:text("new item")')).toBe(true);
expect(await page.isEnabled(':text("Remove All")')).toBe(true);
```
This patch:
- introduces non-exported but used in api/impl struct types (e.g. Point);
- makes all client classes implement respective public api interface.
Pros:
- Typescript is now responsible for type checking.
We can remove our doclint checker (not removed yet).
- Electron and Android types can be defined in the same way
(this is not implemented yet).
- We can move most of the type structs like Point to the public api
and make some of them available.
Cons:
- Any cons?
