[TIDOC-1807, TIDOC-1806] Filter APIs by platform

This PR includes both the UI (TIDOC-1806) and logic (TIDOC-1807) commits for API filtering.

Generally expected behavior:
* If all platforms are "checked", there should be no change from the current/published site's behavior.
* If all platforms are un-checked, it should be equivalent to typing a non-matching term in the 'Filter class members' box.
* For class properties/methods that don't specify a `platforms` property in the YML source, it's assumed they support ALL platforms. This is often the case for inherited members. 

In general, play around and see if the behavior you get is expected.