Improved shortcut behavior

When we successfully execute a shortcut, we shouldn't let anything else
happen in the event path.  Therefore, I stop the key-press-event if it
is successfully handled.

This patch also adds shortcuts for seeking during playback.

ocarina/components/entry.py

@@ -34,3 +34,11 @@ def key_press(widget, event):
 		LS.controls.next()
 	elif name == "s":
 		LS.controls.stop()
+	elif name == "Right":
+		LS.controls.seek_forward()
+	elif name == "Left":
+		LS.controls.seek_backward()
+	else:
+		return False
+	widget.emit_stop_by_name("key-press-event")
+	return True