Fixed input accessory crashes

There are two possible causes for crashes because of `SLKInputAccessoryView`:

- The handling of adding/removing the keyPath observer was very sloppy. Try-catch?!  
Cleaned up.
- `SLKTextViewController` used the selector `inputAccessoryView` for handling its custom input accessory. However the name collides with `UIResponder` and internal `UIViewController` logic causing a crash when leaving the screen while the keyboard is up.  
Renamed to `slk_inputAccessoryView`.