Add appstream metainfo file + screenshot

[PunkPangolin]

Add io.ocrmypdf.ocrmypdf.metainfo.xml to help with #1461

There is also a screenshot required to be found on Flathub.org search, so I added a basic one

~~src/ocrmypdf/data is the most common path for metainfo files, but if you prefer, I can also place it somewhere else.~~

Placed it into moved it into /misc/flatpak to not clutter the data folder with a screenshot.

I apologize for the number of commits, please squash this when merging.