Reverse channels button

This replaces the ambiguous "Rev." checkbox with a reverse button (discussed in https://github.com/TauLabs/TauLabs/issues/258). The checkbox's secondary purpose, providing a visual clue for easily identifying which channels are reversed, is replaced by a slider that reverses the fill direction.

By removing the checkbox, we get a bit of extra space in the input tab. Note that the original checkbox was never user-checkable, so it was purely ornamental.
![screen shot 2013-07-20 at 8 31 22 am](https://f.cloud.github.com/assets/1118185/829755/cd437434-f0fd-11e2-93b8-9230f456af08.png)

On the output tab, we lose some space, but this was in some sense inevitable since "Rev." was ambiguous and would eventually need to be changed to "Reverse" in order to have a more intuitive interface. I need to examine why the spacing between output sliders has grown a little larger, but I'll handle that at the same time as the larger question of why the output sliders require so much vertical space vs. the input sliders.
![screen shot 2013-07-20 at 8 31 49 am](https://f.cloud.github.com/assets/1118185/829756/cd5cc8da-f0fd-11e2-8ff8-db01ee6d198b.png)

This needs to be tested on Windows and Linux to ensure that the stylesheets behave identically across platforms.

Fixes #258.