This removes old icons included with the applet and instead uses icons from the current theme. Known issues: * Loopback interface does not have an icon, instead we use a "reload" icon to simulate it. * PLIP/SLIP interfaces do not have proper icons, I managed to find an old PLIP icon in the hicolor theme and re-purposed it. * Since not all icons have the same levels of scaling, I'm capping the size at 48px to keep all interfaces consistent. * There are no icons for Wireless Quality, so I retained the existing ones for now.