Kill RenderViewItem, and move contents to the WebView instead

[Chris Coulson]

WebView is currently a QQuickItem with no contents and no event handling. Contents and event handling are provided by child QQuickItem's (RenderViewItem), which have a corresponding RenderWidgetHostView. This is analogous to Chromium Aura, where each RenderWidgetHostView has a corresponding aura::Window.

However, we should try to get rid of these child QQuickItem's. WebView should provide contents and event handling itself, and we should stop subclassing RenderWidgetHostView in qt/core. This would allow us to have a browser-side compositor for the webview, much like ContentViewCore / ContentViewRenderView on Android