Improve images and screenshots

[Caerbannog]

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I suppose we could do a better job at keeping images consistent with each other in a single document, and updated with the latest KiCad version. My proposals:

1. Icons can be exported directly from the KiCad sources. Currently they are either mixed with screenshots under src/<DOCUMENT>/images/ or in a separate src/<DOCUMENT>/images/icons. Proposal: move all of them under src/icons to avoid duplication and make sure they keep their original name.

2. The screenshots are usually arbitrary, so updating them while keeping the meaning isn't easy. Proposal: use always the same example project, like the hackRF demo. Or a specific board commited here, with all features. That's lot of work.

3. Screenshots have names like 100000000000010700000125A4376EBB that probably date from the ODT export. We could give them proper names.

   Maybe try to name them with instructions on how they where made like eeschema-hackrf-con1-contextmenu.png.

4. The PDF output sometimes has image DPI inconsistencies that @ciampix addressed in src/AUTHORS_README.adoc, someone should have a look at it.

5. Minor subject: OS and window manager inconsistencies.

   What are your ideas on these and other subjects?