Add splash screen for iOS

These work well on my iPad 4 but remain to be tested on other devices.
The arrow on the portrait splash screen serves as a friendly reminder
that the application works best in landscape orientation!
Scale to high-DPI devices

Devices with much higher pixel densities (AKA 'retina displays')
typically use different logical/CSS units, in order to make things
easier for designers. Unfortunately, the exact behaviour is not
standardised and varies between vendors (of both operating systems
and browsers); making it slightly trickier than it perhaps should be...

The new setting `autoScale` allows this functionality to be disabled.