Set ::visible to false on panels when they are destroyed

spec/panel-spec.coffee

@@ -21,3 +21,7 @@ describe "Panel", ->
       panel.show()
       expect(panel.isVisible()).toBe true
       expect(spy).toHaveBeenCalledWith(true)
+
+      panel.destroy()
+      expect(panel.isVisible()).toBe false
+      expect(spy).toHaveBeenCalledWith(false)

src/panel.coffee

@@ -21,6 +21,7 @@ class Panel
 
   # Public: Destroy and remove this panel from the UI.
   destroy: ->
+    @hide()
     @emitter.emit 'did-destroy', this
     @emitter.dispose()