TechDraw: Default position + snapping of dimensions

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Problem description

Following up on https://github.com/FreeCAD/FreeCAD/pull/13659

Currently, a new dimension is placed directly at the position between the selected features:

Ideally, the default location is 10 mm outside of the models edges and it should be possible to snap to this location when dragging.

1. create dimension at default at a distance of the models selected outline, in outward direction of the model. If there is already a dimension, increase this by the cascading dimension distance setting (default 7mm).
2. Allow snapping when dragging the dimension inside and outside of the selected model line with a distance of 10 mm which was used for the dimension for distances and angles.


@PaddleStroke FYI

### Full version info

```shell
OS: Windows 11 build 22631
Word size of FreeCAD: 64-bit
Version: 0.22.0dev.37063 (Git)
Build type: Release
Branch: main
Hash: 680792030fd664c94829e2c1b78b21788e8d9879
Python 3.11.9, Qt 5.15.13, Coin 4.0.2, Vtk 9.2.6, OCC 7.7.2
Locale: German/Germany (de_DE)
Installed mods:
* BIM 2021.12.0
* CfdOF 1.25.11
* Curves 0.6.35
* dodo 1.0.1
* fasteners 0.5.20
* freecad.gears 1.2.0
* OpenTheme 2024.5.3
* OpticsWorkbench 1.0.17
* sheetmetal 0.4.13
```

### Subproject(s) affected?

Techdraw

### Anything else?

Related issue:
- https://github.com/FreeCAD/FreeCAD/issues/11636

### Code of Conduct

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