A more compact DateInput on GTK+ Backend

### What is the problem or limitation you are having?

The GTK DateInput is different from all other DateInputs because it displays a larger calendar, rather than a compact line with the date; however, in some contexts one may except a short line displaying the date, such that the layout differences are not that large between platforms.

### Describe the solution you'd like

Not clear right now. I'm not sure if any more compact solution from which the calendar displays in a dialog would feel non-native; investigation of date related apps on Linux is needed before we should make such a decision.

### Describe alternatives you've considered

Status quo, users can if-condition to switch their interface to a button with a popup calendar for GTK only if needed; however I'm feeling that there's *too* large of a layout difference when it comes to date picking for Linux compared to other platforms.

### Additional context

The latest PR adds a success story for an app which uses a TextInput to pick a date:

However, one may imagine DateInput being used there. That's where I got the inspiration for this issue, which may be *totally* flawed.

This might just be a stupid thing that I can't let go of...

------

### Edit

Another use case of this would be if you have something like
```
Date: [DATEINPUT]
```
then the rather large DATEINPUT would be out of place compared to the label.

Start with the existing GTK DateInput implementation and investigate date-related Linux applications, as the issue suggests, to identify whether a compact native pattern is appropriate. Done means reaching agreement on a concrete compact design and its behavior; the issue currently does not name files, tests, or an implementation approach.

Written by the indexing model from the issue text.