* Make core Ticker abstract and public
Make the core Ticker abstract and public so it can be inherited by platform
implementations; remove now-unused parts of original Ticker; add CreateTicker
to IPlatformServices; remove unused CreateTimer methods from IPlatformServices
* Add docs for Ticker
* Remove unnecessary Ticker.Default set
* Move Ticker into Internals
* Update Ticker docs
* Remove old Ticker docs
* Remove commented code