demo: Move initial help text to a div

Current OpenSeadragon doesn't have _setMessage().
demo: Update to OpenSeadragon 0.9.127
demo: OpenSeadragon configuration tweaks

- Don't show extra border or allow zooming out further than required
- Allow zooming to 2x highest-resolution level
- Load an extra level of tiles for improved image quality
- Don't load lowest-resolution pyramid levels
demo: Fix fullscreen mode
demo: Update OpenSeadragon URL
demo: Drop obsolete workaround for OpenSeadragon timer leak
demo: Update to jQuery 1.9.1
demo: Reduce font size of initial help text
demo: Work around blurry viewport after loading slide