Unify name of placeholder icon and use proper icon instead of empty png.

[Findus23]

At the moment the placeholder icon (Transparent png) has multiple different names, which is bugging me for some time.

placeholder_icon_filenames = {
    "brand": "unk.png",
    "browsers": "UNK.png",
    "devices": "unknown.png",
    "os": "UNK.png",
    "searchEngines": "xx.png",
    "socials": "xx.png"
}

It would be nice to rename them to one common name (e.g. unk.png).

This also needs to be changed in Piwik (functions for generating icon path and in tests) and in the ReferrersManager-Plugin (see sgiehl/piwik-plugin-ReferrersManager#11)

[sgiehl]

We should consider replacing the empty images with maybe something else like a question mark or so.
We currently have some places where we show overlays when hovering the images (e.g. in visitor log). Having an empty image makes it difficult to see there is something that can be hovered.

We could also look for good images representing something "unknown" for each type, but maybe that isn't really needed.

[Findus23]

That's a great idea. I'll definitely do this.

representing something "unknown" for each type

While I would really like this, I'm not sure if I'm able to create nice looking ones. Maybe someone who is better at designing than me could help.

[Findus23]

BTW: There is already a proper icon for an unknown flag, so maybe we could create similar ones for the rest:
https://github.com/matomo-org/matomo-icons/blob/master/unk.flag.svg