Destroy folds when DisplayBuffer is destroyed

Fixes #4562

spec/text-editor-spec.coffee

@@ -4013,6 +4013,7 @@ describe "TextEditor", ->
 
   describe ".destroy()", ->
     it "destroys all markers associated with the edit session", ->
+      editor.foldAll()
       expect(buffer.getMarkerCount()).toBeGreaterThan 0
       editor.destroy()
       expect(buffer.getMarkerCount()).toBe 0

src/display-buffer.coffee

@@ -1173,6 +1173,7 @@ class DisplayBuffer extends Model
     return
 
   destroyed: ->
+    fold.destroy() for markerId, fold of @foldsByMarkerId
     marker.disposables.dispose() for id, marker of @markers
     @scopedConfigSubscriptions.dispose()
     @disposables.dispose()