Add style sheet for inverted horizontal sliders.
Refactored input and output channel reversing. This involved
simplifying the code, as well as removing checkboxes and
replacing with less ambiguous visual clues.
Remove unused reverse button.
Changed behavior so that the spin boxes would wait for the user to finish
before updating the neutral slider.
Added swap icon.
Added reverse icon button.
Merge pull request #760 from kubark42/reverse-channels-button

Reverse channels button