Update Imagery Layers Manipulation demo to HTML5 sliders.

Remove Dojo from another demo.  The diffs should be smaller if you ignore whitespace, since the indentation changed.

Also, this exposed an odd Sandcastle bug, where the Unicode characters `&#9650;` and `&#9660;` get HTML-decoded when you "Save As..." from the Sandcastle editor.  I'll write a separate issue for that but I don't think it needs to hold up 1.0 or this PR.

/cc #1083.