fix(content): use handlers that actually exist in event listeners (#2131)
fix(content): use handlers that actually exist in event listeners (#2131)

* fix(content): use handlers that actually exist in event listeners

In a recent change, handlers now only exist in `Content#handlers`, but there were still a couple of places where they were being dereferenced directly from the instance. This was causing some side effects in iOS.

Fixes #2125
Fixes #2129

* fix(content): also fix removeEventListeners

ianstormtaylor/slate,ianstormtaylor/slate,isubastiCadmus/slate