Extract insertBr() functionality into a proper "InsertLineBreak" command

In my deployment of TInyMCE I have a menu item for "Line Break" that should cause consistent behavior to e.g. pressing shift-enter in the editor. It seems like it makes sense, especially given that "InsertLineBreak" is a standard DOM editing command, that the behavior from EnterKey.js be formalized as the standard for TinyMCE. This would also give clients the ability to for example override it from a plugin if desired, to call through to it from plugins, etc.