[Feature Request] Expressions: pick an object to add a reference

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Problem description

Expressions usage could be easier if we could pick an object in the tree or the 3D view to add a reference to it.
If **[Feature Request] Sketcher: show dimensions out of edit mode #12145** is implemented it can concern sketches dimensional constraints.

### Full version info

```shell
OS: Debian GNU/Linux trixie/sid (XFCE/xfce)
Word size of FreeCAD: 64-bit
Version: 0.22.0dev.35726 +1 (Git)
Build type: Release
Branch: short_suppress
Hash: c61705227a9b8c149bcf33e665846ddc9dc79eb0
Python 3.11.7, Qt 5.15.10, Coin 4.0.2, Vtk 9.1.0, OCC 7.6.3
Locale: French/France (fr_FR)
```

### Subproject(s) affected?

Expressions

### Anything else?

_No response_

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct

Start by reviewing the Expressions subsystem and how object selection works in the tree and 3D view. Compare the related Sketcher dimensional-constraints issue #12145. Done means a user can pick an object to add its reference to an expression, with the sketch-constraint case clarified if that issue is implemented.

Written by the indexing model from the issue text.