Issue #3011 - Part 1: Add As{Text|Html}Editor() and AsEditorBase()

This adds helper functions to get specific types of content editors (if
possible) through the nsIEditor interface, as opposed to doing manual
casting and the ad hoc `GetHTMLEditor()` function.

Helper functions are spread out over multiple headers due to the
circular dependency issues that would be triggered otherwise.

editor/libeditor/EditorBase.cpp

@@ -142,6 +142,7 @@ EditorBase::EditorBase()
   , mDispatchInputEvent(true)
   , mIsInEditAction(false)
   , mHidingCaret(false)
+  , mIsHTMLEditorClass(false)
 {
 }