[plugin] Panorama Viewer

[Aerilius <aerilius gmail com>]

I developed a panorama viewer plugin that detects xmp GPano tags in photos taken by mobile phones or stitched with Hugin. In that case, it displays the panorama projected onto a sphere. 

Panoramas are nowadays common elements in photo collections, so when I presented holiday photos it was a mediocre experience zooming and panning over a huge distorted photo in Eye of Gnome, whereas most web platforms (Google Photos, FB) would be so smart and show it in 360°. Something that was missing from eog.

Repository:

    https://github.com/Aerilius/eog_panorama

For now I am using photo-sphere-viewer.js in a webview. It is not ideal because it takes much memory and pushing local files into the webview is slow. I'm curious whether Gtk scene graph kit is a better native solution somewhen in future.

- Are there any problems when I temporarily remove EogScrollView from the widget hierarchy to replace it with the panorama widget? (I could not get it work by just hiding.)

- For reading xmp metadata I currently use exiftool, but it is not included in distros by default. Is there such a function in eog that I could use instead, to get rid of the dependency?

I would be very happy about feedback.