Notifications.

And a cosmetic change to the graph.
It adds a dependency to libnotify (installed with every decent Linux DE)
A notification is show whenever a download finishes, or a magnet download starts.

A few more settings have been added for the graph, you can draw a dashed curve instead of normal curves, and you can fill them with a color, the fill colors will be 50% transparent to keep the two curves visible.

See #111