Viewer: make viewport messages private, repeatable
Viewer: make viewport messages private, repeatable

* Marked _showMessage/_hideMessage as private
* Fix DOM manipulation bug: call standard removeNode() method rather
  than proprietary WebKit .remove()
* Change messageDiv to be the outer wrapper to simplify removing the
  entire message container

seanich/openseadragon,TMUND/openseadragon,msalsbery/openseadragon,rdlester/openseadragon,openseadragon/openseadragon,LarissaSmith/openseadragon,jmnetus/openseadragon,lidasong2014/openseadragon,LarissaSmith/openseadragon,Iced-Tea/openseadragon,sul-dlss/openseadragon,avandecreme/openseadragon,janelia-flyem/openseadragon,openseadragon/openseadragon,janelia-flyem/openseadragon,mcanthony/openseadragon,goofwear/openseadragon,ricochet2200/openseadragon,avandecreme/openseadragon,LafayetteCollegeLibraries/openseadragon,mcanthony/openseadragon,jmnetus/openseadragon,rdlester/openseadragon,shaunstanislaus/openseadragon,msalsbery/openseadragon,shaunstanislaus/openseadragon,daverome/openseadragon,TMUND/openseadragon,lidasong2014/openseadragon,goofwear/openseadragon,ricochet2200/openseadragon,daverome/openseadragon,Iced-Tea/openseadragon,seanich/openseadragon