Default ZIM icon (if no icon in the ZIM) is not displayed in tab bar

[kelson42]

It should display the default icon like in the library

Tested with https://library.kiwix.org/?lang=fr&q=ubuntu#lang=fra&q=ubuntu

[kelson42]

@mgautierfr I really wonder if we should not build something at a low level in the libkiwix to provide the placeholder icon... otherwise we might have dozens of such tickets all over the reader landscape.

[kelson42]

@rgaudin @benoit74 I would like your feedback on this last comment... I'm still not 100% sure.

[rgaudin]

Not a replacement of course but we have a lot of things in place to prevent openZIM from creating ZIM without icons now. This ticket's example is quite old