Add app icon

Added the app icon at 16px, 24px, 32px, and scalable. If they need to be tweaked or anything let me know!

@Tamaranch you mentioned other sizes like 48px and 64px but since this isn't really an "app" (like Thunar, Mousepad, etc) and is a plugin, the only place the icons will be seen is in the panel dialogs (24px and 32px), window icon (16px), and the about dialog if it has one (which would use 128px or scalable I think). I can do the other sizes but I'd rather not if they won't be used, so just let me know.

Also if you want I can add the makefiles in the icon dirs (It looks like I should be able to copy/tweak the icon makefiles from other panel plugins). If you'd like me to add them to this PR as additional commits or make another PR for that I can do that.

Thanks.