Commit ddfc7715 changed the
airbrush periodic stamp behavior, so that instead of using the main
brush, it issued a full MOTION event, potentially using a different
brush when using a GIH brush.

Fix this, by renaming the "timeout" signal of GimpAirbrush to
"stamp", and by adding a new gimp_airbrush_stamp() function, which
should be used for painting the periodic airbrush dab in response,
instead of calling gimp_paint_core_paint() directly, and which
calls gimp_airbrush_paint() instead, as the old code did.

In order to call this function from the paint thread, we replace
the various gimp_paint_tool_paint_core_foo() functions, introduced
in the above commit, with a generic gimp_paint_tool_paint_push()
function, which takes a callback (and a data pointer) to run on the
paint thread, and queues it for execution (when not using the paint
thread, the function is called directly from the calling thread.)