More user-friendly icons

I've added unique "off" icons for each of the three overlay filters, replacing the X that showed up before; this should make it easier for new players to figure out which button is which.

I've also completely replaced the "show cones" button with one that's more suggestive of an antenna cone. This will be especially relevant once we fix #13 and cones will no longer work only on planets.

Note: VaporTrail suggested a [better cone icon](http://forum.kerbalspaceprogram.com/threads/83305?p=1447594#post1447594) than what my own art skills could make. However, I don't yet have VaporTrail's explicit permission to use it in RemoteTech. I'd rather merge their icon to `develop` if possible, but I'd rather use my version than not have this be in the 1.5.0 release.