Image addition

[Stuart Langridge]

Allow addition of images to a presentation. Ask for an image URL, then convert the image at that URL to a data URL and embed the data URL in the presentation.

This may only be possible using a web service as a proxy to fetch the image -- an arbitrary page can't fetch arbitrary image URLs and then read their data to convert to a data URL.

Note that images must be added as <h? style="-webkit-transform: translate(150px,105px) scale(0.1); -webkit-transform-origin: 0 0"><img src="data:image/png,%89PNG%0D..."></h?>. The scale is altered by UI from bug 435752. When initially imported, calculate a scale which makes the image small enough to fit reasonably on the page -- say, make it the height of h1 text.