Re-add GPS EXIF tool for auto-placing images using embedded GPS EXIF data

[jywarren]

#142 introduced this awesome feature, which uses image metadata to try to place images roughly over their real-world positions on the globe. Compass data is also used to try to guess appropriate image rotation.

Unfortunately this feature is not shown in the demo and i think it may need a little help to get featured more prominently and/or debugged; it's not in the README either:

Ideally it should be present in the default toolbar and:

• should be enabled if EXIF data is present
• should show a tooltip No GPS EXIF metadata found if not
• should be documented in the README
• should be covered by a test (using sample images below! Note that EXIF tags should survive an image resize so if we want a test image we can use a small one for efficiency.)

We had used a crosshairs icon in the past for this tool.

Example images and known expected locations

I think a solution for this is already up at #421 but may need updating/refactoring. Sample images and a sample use case can be found at https://publiclab.org/notes/patcoyle/03-23-2015/using-the-mapknitter-2-image-placement-using-geotagged-images and should lead to placement as shown here:

Related code links

Noting related code not only at the PR above (#421) but also at:

• https://github.com/publiclab/Leaflet.DistortableImage/blob/a980773b0ea20f97c90d491b6b44861c48f5a0b4/src/edit/getEXIFdata.js

Thanks all!

[jywarren]

Moving to #1024