[embedlite] Move single tap handling of input to JavaScript

This adds basic cases. We need to add support for viewport meta tag.
Also basic maximum scale factor is missing.

In addition, path for viewport move/scale should be calculated so
that it would be the shortest possible path where input would be
fully visible. Currently there are hotspots in each corner.

Biggest problem is that we get "Viewport:Changed" message
only after content is moved a bit.