[DevTools] Apps implementation.

DevTools frontend now runs an app: screencast app, simple undocked app or advanced app with docking support.
There will be a toolbox app soon (the always-docked part of DevTools).

This patch mostly moves the code into the new WebInspector.App hierarchy.

BUG=none
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/307623003

git-svn-id: svn://svn.chromium.org/blink/trunk@175060 bbb929c8-8fbe-4397-9dbb-9b2b20218538