Improve Icon Selection/Upload

I think we should be able to optionally set icons for all blocks, not just the promoted ones.
That way we could style the 'Other' blocks tab just like the 'Promoted' blocks tab. Both provide the user the option to add a block but both look different right now and that's confusing UX.

flyke → changed the visibility of the branch 3404515-improve-icon-selectionupload to hidden.

Added a patch that works on 3.1.x.

I removed the css adjustments in the patch so that this patch has no conflict with patch from #3469286 ✨ Navigation module integration Needs work .

Added screenshots and a screen recording of how it looks with this patch if you give all your blocks an icon.

Since I removed css adjustments from this patch, here is an example of custom css I add for now via a custom module:

.toolbar-plus-sidebar {
    --admin-toolbar-z-index: 501;
    --admin-toolbar-color-white: rgba(var(--bs-blue-rgb), 1);
    --admin-toolbar-font-family: inter, sans-serif;
    --admin-toolbar-color-gray-100: #d8dfea;
    color: white;
}

#promoted-block-content, #other-block-content {
    margin: 0 1em;
}

.draggable-block-list .draggable-block {
    margin: 1em 0;
}

.draggable-block-list .draggable-block-image {
    padding: 2em;
    text-align: center;
    display: block;
    background-repeat: no-repeat;
    background-position: center;
    background-size: contain;
}

.block-categories .block-category-details {
    background: none;
}

#other-block-content details {
    border: 1px solid rgba(255, 255, 255, .2);
    box-shadow: none;
}

#other-block-content .details-wrapper {
    margin: 1em 0;
    padding: 0;
    color: white;
}

The subquestion to edit icon for all blocks is resolved. It seems you can now set icon for every possible block at /admin/structure/block/icons.

However, the original issue was a request to be able to upload a file for an icon. Maybe that should still be discussed ? For my opinion it is not necessary, but it is something you can do for Paragraphs: when you create/edit a paragraph then you have the option to either set a path or upload a file for the icon. It makes sense to apply the same options here.