Is the next after_window_pop triggered by a call to pop_window?

Otherwise, it's by the user via the navigation bar "Back" button.

Property definitions

ios :: ui $ App :: manual_pop=
	# Is the next `after_window_pop`  triggered by a call to `pop_window`?
	#
	# Otherwise, it's by the user via the navigation bar "Back" button.
	private var manual_pop = false
lib/ios/ui/ui.nit:145,2--148,31