Propagate activate/deactivate to subviews on iOS (issue 719)

This will manually walk the subviews of a view controller and propagate the activation/deactivation state of that view controller. Note that this does not address how activation/deactivation should work for table/collection cells, but this is definitely a step forward.

Fixes #735
Would `ICanForceManualActivation` be a better name for this?