This will allow us to share code with LibWebView from SerenityOS.
(This would otherwise not work, since its "WebView" namespace collides
with our "WebView" class.)
Also, we should eventually move towards a more sophisticated
multi-process WebView like OOPWV.
This patch removes the dual-event-loop setup, leaving only the Qt event
loop. We teach LibWeb how to drive Qt by installing an EventLoopPlugin.
This removes the 50ms latency on all UI interactions (and network
requests, etc.)
This patch adds an event handler to the main window which allows it to
respond to a user closing the window. This event is then passed on to
the LibCore event loop, which allows the application quit itself.
Previously the application would hang, only running in the background,
until killed by an external force.
