Add a Debounce Extension to DispatcherTimer

[michael-hawker]

For rate-limiting UI events (like triggering on typing)
For use in Graph Controls later

Issue: #2989

PR Type

What kind of change does this PR introduce?

• Feature

What is the current behavior?

It's hard to know when the user is typing when to execute a search query.

What is the new behavior?

By wrapping typed event callback in debounce method, it's easy to only query a remote server after the user has delayed in their typing.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

Other information

[pedasmith]

The documentation for this doesn't seem to have made it into https://docs.microsoft.com/en-us/dotnet/api/microsoft.toolkit.uwp.ui.extensions?view=win-comm-toolkit-dotnet-stable

[michael-hawker]

@pedasmith yeah, we had an issue updating the .NET docs in our last release. We just sorted out other issues with our doc building, so that's the next doc issue on my list to resolve still. Sorry for the trouble, the docs are just generated from the XML comments here, so they're still accurate.

I opened an Issue on our Doc Repo here for tracking.