Experimental app list: Added "All apps" button on start page.

This replaces the fifth app icon on the start page. Clicking it
transitions to the apps grid view.

Currently has no icon, and it's a simple slide transition, but
eventually it will have a folder-like icon with a folder open animation.


Review URL: https://codereview.chromium.org/665233002

Cr-Commit-Position: refs/heads/master@{#301778}
12 files changed