Added iPad icons

• New `Icon-76@2x.png` and `Icon-76.png` are downscaled from the 180x180 `Icon-60@3x.png`.
	» The non-retina `-76` variant is enough of a downsample ratio that it looks great; the `-76@2x` variant however is a touch blurry and will need to be re-created from the original assets at some point.
• New `Icon-Small-40.png` and `Icon-Small.png` created via a direct 1/2x downscale of `Icon-Small-40@2x.png` and `Icon-Small-40.png`.
	» Since it's an even ratio, there should be no visual difference from these and ones that would be created from a higher-res/vector source.
• Updated `AppIcon.appiconset` assignments.