Change "Show Private" to "Show Protected" and never show private members

[kevinpschaaf]

Description

The "Show Private" checkbox currently seems to toggle methods with _, which generally should be considered protected per internal polymer conventions. Using the new criteria discussed in https://github.com/Polymer/polymer-analyzer/issues/586 (which gives priority to @private/@protected/@public annotations and falls back to inference based on _'s), the toggle should toggle showing protected members (and never show private members).

Additionally, we should consider whether the default should be to show or hide protected members, since e.g. mixins, behaviors, and base classes like Polymer.Element largely consist of protected members for use by subclassers. Perhaps the default for elements could be "Show Protected: unchecked" and for everything else "Show Protected: checked". Seems like it would be nice to sort the members by public/protected as well, but that's maybe more arguable.