Extracting images from their base64 encoded form into pngs for easier maintenance

Hello again!

In my tinkering I found I needed to change some of the images baked in to the CSS, so I decoded them, modified them in photoshop, and then re-encoded them and pasted it back in to the CSS. This is a pretty slow workflow, so I wrote this patch to speed things up and help with maintainability. 

I decoded all the PNGs in src/bbui.css from base64 and saved them to folders named according to their parent selector, then generated a new CSS file with filepaths in place of the base64 data. 

At build time images are encoded in to base64 by the build script so the output bbui.css in /pkg and /samples is identical to what it was before.

None of the images in this patch are modified from their original form, they were just straight transcoded from base64 to PNG.