cancel reverts changes; retains callback functionalty

Hi Jean,

I noticed ui pref dialog Cancel button didn't restore Interface tab changes, so I've been playing around a bit with alternate interpretations of ui pref dialog cancel behavior, including an apply button.

There are several branches to that effect. Merging is pretty flexible, and can be done incrementally both with, and without audio functionality.

The branches ending with _m are without audio. All other branches are spawned off of wave-output branch.

Correspondingly named audio branches can be merged into non audio _m branches. Branches with names containing '_compat', retain dialog Interface tab callback functionality.

For instance, cancel-compat-m ('_compat' branches retain interface tab widget event callback functionality) merged into master, dismisses the dialog, and restores to settings before dialog opened. The restoration includes immediate changes via Interface tab callbacks.

Another for instance - you can merge a non audio '_m' branch to master, then merge a correspondingly named audio branch to master, or merge to wave-audio intermediately, and then wave-audio to master.

Cheers!