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.
On high-dpi systems QT will automatically scale up painting operations.
This results in an ugly blurry browser window surrounded by a nicely
scaled window frame. This patch applies an inverse scale to the WebView
widget, ensuring the painting and mouse events are lined up and draw
with a 1:1 pixel ratio with respect to the display's device pixels.
