Convert size from app.nit relative size to iOS font points

Property definitions

ios :: ui $ TextView :: ios_points
	# Convert `size` from app.nit relative size to iOS font points
	private fun ios_points(size: nullable Float): Float
	do
		size = size or else 1.0
		return 8.0 + size * 5.0
	end
lib/ios/ui/ui.nit:267,2--272,4