Re-introduce the local pointer to the Annotator class

Partially revert the changes made in https://github.com/hypothesis/h/commit/c840a620fc040ceb7941c9b50a379b9c271b0728. Fixes #1201.

h/js/guest.coffee

@@ -466,7 +466,7 @@ class Annotator.Guest extends Annotator
     # We have to clear the selection.
     # (Annotator does this automatically by focusing on
     # one of the input fields in the editor.)
-    Annotator.util.getGlobal().getSelection().removeAllRanges()
+    @Annotator.util.getGlobal().getSelection().removeAllRanges()
 
   onSetTool: (name) ->
     switch name

h/js/host.coffee

@@ -36,6 +36,10 @@ class Annotator.Host extends Annotator.Guest
     # Scan the document
     this.scanDocument "Host initialized"
 
+    # Save this reference to the Annotator class, so it's available
+    # later, even if someone has deleted the original reference
+    @Annotator = Annotator
+
   _setupXDM: (options) ->
     channel = super