print operation: let subclasses use a custom paginate

print operation: let subclasses use a custom paginate

GtkPrintOperation was emitting paginate only if a signal was
connected, this meant that subclassing and overriding the
paginate vfunc lead to the unexpected result that paginate did
not run.
Instead we always emit the signal and use a custom accumulator:
if there is a signal we just run that and avoid the default
handler, otherwise we run the default handler which can be the
one by the subclass or the default handler that just skips
pagination.

Patch by Yevgen Muntyan, fixes #345345