There's no point in busy-waiting for the condition to come true.
By passing the `WaitForMoreEvents` flag to `processEvents()`, we allow
Qt to block until it has something for us to react to.
This was extremely noticeable when waiting for large resources to
finish loading.
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.)