Retain Cycle Fixes

[dzenbot]

For some yet unknown reasons, registering UIKeyCommand objects from within SLKTextViewController caused the view controller never to be deallocated, when interacting with at least 1 of these commands on the iPad. Certainly due to a retain cycle since this didn't cause any memory leaks. With the right repro steps, it was possible to get incremental retain cycles.

After profiling this for a full day, and looking for different alternatives, I decided to move ALL the key command registration back to SLKTextView, using a simpler-to-use API like:

- (void)observeKeyInput:(NSString *)input
              modifiers:(UIKeyModifierFlags)modifiers
                  title:(NSString *)title
             completion:(void (^)(UIKeyCommand *keyCommand))completion;

Key commands are then registered internally in SLKTextView, using a common selector and forwarding the events to the block callbacks. By doing this, the view controller is out of risk from being retained, possibly by a UIKit private object since it was untraceable. It seems to me, that this is a behavior change on UIKit since iOS 9, but unsure.

[sukeban]

This code looks good. Such an annoying bug!