Refactor dialog implementation

[freakboy3742]

In the process of auditing WebView, I noticed an opportunity for code reuse in Dialog.

This refactors the Dialogs implementation so that:

1. There's a clear interface/impl separation for dialog objects
2. There's a full interface/_impl/native chain. This should make dialog testing simpler (hopefully?)
3. The asynchronous result capability is factored out as a base class that can be re-used

Testing regimen for this is the dialogs example.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct