RelocatableNode for Inline and Block Images

HTML5's drag and drop has a ton of benefits and a couple of
limitations. To achieve a native drag marker, an image tag helper
is used to indicate the size of the relocatable node. Chrome
shades the marker gray natively, Firefox is styled to match.

Change-Id: I755b698a3d968cc7e6ff125109d68ac83fd8a8a2
Merge "RelocatableNode for Inline and Block Images"