Inputmethods API

This adds `setDisabledInputMethods()` to the public API, which enables input methods to be enabled/disabled during the lifecycle of a scroller. With this, I can set up a scroller, disable all use inputs and scroll the pages myself manually (using `scrollTo`, etc.), but occasionally enable user interactions to work on a single page transition.

I found `_toggleEventHandlers` very confusing, and almost impossible to rework with the new API flexibility in mind, so I split it into `set...` and `remove...` methods, which is cleaner and easier to understand.