Fix invocations of $::application->yield

I had added invocations of $::application->yield to PDL::Graphics::Prima's
on_changetitle and PDL::Graphics::Prima::Axis's repaint_parent methods.
These broke animations in one of my applications due to deep recursion of
some sort. This commit makes those yields conditioned on the definedness of
$PERLDL::TERM, the only plausible situation in which we would need such
immediate updates.
Merge branch 'master' of github.com:run4flat/PDL-Graphics-Prima