Dock: If only a subtitle is enabled, display it styled as a title

### Description of the new feature / enhancement

The item subtitle may actually display more useful information, so this combination can make sense. Since there is no title, there is no visual hierarchy for the subtitle to follow, and subtitle alone looks misplaced.

### Scenario when this would be used?

UX

### Supporting information

_No response_

Start by locating the Dock implementation responsible for displaying an item's title and subtitle, then compare the two shown subtitle-only layouts. Done means that an item with only a subtitle presents it with title styling and an appropriate visual hierarchy, while existing title-and-subtitle behavior remains unchanged.

Written by the indexing model from the issue text.