Extract EXIF data when adding pictures

[clochix]

Some code has been added to Photos to extract GPS information from the EXIF data of photos. Other EXIF data, like orientation, may be useful to other applications (for example, for displaying an image gallery in Files). So extracting and saving EXIF data should probably take place into the data-system.

[babolivier]

As discussed on IRC, as @flyingrub has also been added ID3 tags extraction to the DataSystem, I think it'd be wise to create a "submodule", or module, or library (call it whatever you want), some place in the DataSystem (or in an external NPM module) focused only on metadata extraction. It'll then be easier to add metadata extraction for other document types if we realise we need/want it (we can even imagine a factory architecture for that, returning an extractor object depending on the document's MIME type, but that may be only me getting too much excited :wink: ).