Option to use the entry type's icon as a thumbnail when an entry has none

[thupsi]

Builds on #20156 (Thumbnail Source Candidates), which lets an entry type name several thumbnail sources and use the first one with a value.

The idea

Add an optional setting next to the thumbnail source(s): Use the entry type's icon when there's no thumbnail. When it's on and none of the thumbnail sources has a value for an entry, its card or chip shows the entry type's icon as the thumbnail, instead of having no thumbnail at all.

It's off by default, so nothing changes for existing entry types.

Why

With #20156, an entry can still run out of sources, for example when it has neither a teaser image nor a hero image. It then renders with no thumbnail in cards and chips. In a mixed list this means:

• Rows with an image and rows without one don't line up, because the thumbnail column is there for some and empty for others.
• An entry type with no meaningful thumbnail at all (a page with no media, a simple data entry) has no visual anchor in an index that otherwise shows thumbnails.

Entry types already have an icon (Entry::getIcon() returns it), so there is already something to show. It tells the editor what kind of entry they're looking at, even without an image.

How it could look

• A checkbox in the field layout's thumbnail settings, under the thumbnail sources.
• The icon shown in the thumbnail slot of cards and chips (a neutral tile, or tinted with the entry type's color if it has one).
• Nothing extra when the entry type has no icon set, so those entries still show no thumbnail.

[thupsi]

Related #20210

[mmikkel]

Big fan of this idea, but also: #20232