Improve the new wireless icons

I made some further improvement to my new icons for wifi. The bars on the previous ones weren't transparent, so they didn't stand out well with certain background colors. I also changed the shape of the icons slightly. 

I'm thinking about making wired connection icons to better match these. Let me know if you have thoughts about that. Feel free to pull this branch if you like the changes, or close it if you don't. 

I am releasing these icons under [CC0](http://creativecommons.org/publicdomain/zero/1.0/#) (public domain). I can supply the svg sources if you want them.