Introduce real 'show always thumbnails' (#298)

@Tamaranch, possibly of interest for you, since related to thumbnailing

Large files anyhow can be excluded via file-size limitation in thunar, so IMO no need to totally drop thumbnailing support for mtp/Gphoto2

If the user selects show thumbnails=Always, then IMO we should always show thumbnails. Totally skipping specific locations, based on bandwidth worries imo is no good idea, since it will not generate the user-expected result / the user will blame the tool to not work as expected.

@hornschorsch If you can, it would be great if you could give the patch a spin.

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