Add a GimpGui::wait() virtual function, and a corresponding
gimp_wait() function.  The function takes an object implementing
the GimpWaitable interface, and a printf-style message, and waits
for the object to become ready, displaying the message as
indication in the meantime.  The default implementation simply
prints the message to STDERR.

Implement the function in gui-vtable, using the busy-dialog plug-
in added in the previous commit, to display the message in a
dialog.  Additionally, if the object implements the GimpCancelable
interface, provide a "cancel" button in the dialog, which, when
pressed, causes gimp_cancelable_cancel() to be called on the
object.  Note that the function keeps waiting on the object even
after requesting cancelation; GimpTriviallyCancelableWaitable can
be used to stop the wait once cancelation has been requested.