Formalise the Sketcher solve(), update and signaling

Issue imported from https://tracker.freecad.org/view.php?id=4519

* **Reporter:** abdullah
* **Date submitted:** 12/24/2020
* **FreeCAD version:**
* **Category:** Feature
* **Status:** assigned
* **Tags:**

# Original report text

In the meantime, rethink the solverNeedsUpdate mechanism, and maybe work at solver level with non-cloned geometry pointers.

Rethink geometry and constraints update before starting dragging operation.

# Other bug information

* **Priority:** normal
* **Severity:** minor
* **Category:** Feature
* **Updated:** 2/6/2021