GimpAirbrush currently performs painting and flushes the image on
its own during the airbrush timeout.  This is unsafe w.r.t. the
paint thread, since the timeout is run on the main thread, while
paint commands should run on the paint thread.

Add a "timeout" signal to GimpAirbrush, and simply emit this signal
during the airbrush timeout, rather than actually painting.

Connect to this signal in GimpAirbrushTool, and use
gimppaintool-paint to perform the actual painting, in a thread-safe
manner (see the previous commit.)