Continue downloads in the background when the window is closed

If downloads are occuring when the browser window is closed, Midori should continue those downloads in the background. This way users can safely close the window and open it to come back to it later without being bothered about downloads still occuring. When the download completes, the app should quit completely.

This helps further push us into the app-resumes-where-it-left-off territory and makes it so users can close the app at any time while allowing task to continue, much like Noise.