Proposed UX enhancement

When swiping down in the top toolbar area, expose all tabs (similar to mobile browsers like Chrome).

Steps to QA:

1. Open a few article tabs
2. Scroll down slightly in the current article tab view. Then scroll all the way back to the top of the article and keep pulling down over the article content area. It is expected that the article will reload.
3. Swipe down over toolbar area (~80dp at top of screen). It is expected that the exploded tab overview will show (all tabs become visible).

Expected step 2 (refreshes page):	Expected step 3:

Note - Updated the task description to better reflect actual expected behavior, which is that the tab overview is only shown when swiping down over the app toolbar, since pulling down in the main section to the top should still refresh the article (as per standard browser behavior).