fix tooltip on processor button widget
add padding to in-line plugin controls
fix detection of aubio library - aubio-0.4 breaks API and ABI.

ideally we'll want atleast_version='0.3.2', max_version='0.3.9'
(or anything before 0.4.0) for the time being.
but waf/autowaf don't seem to allow that.

aubio-0.3.2 was the previous minimum requirement and is
also the last release of audio-0.3.X, so we're good with
exact_version for now.