Sketcher: Constraint icons and fonts in 3D View

===============================================

Based on PRs #4146 #4155.

PR #4155 proposes a scaling factor to be used to scale up constraint icons and the subindex font of icons. The
scaling factor is a parameter.

PR #4146 proposes to derive the constraint icon size from the system font size via function of the dpi. The
constraint icon subindex is a factor of the constraint size.

Observations:
- PR #4146 identifies the need for a scaling factor too, but this is a hardcoded 1.25 factor.
- PR #4146 appears to mix font points and font pixels when deriving the sizes.
- PR #4155 deals exclusively with icon size and subindex font, not with constraint label

Useful concepts:
- Font point is a physical distance. There are 72 points in one inch.
- Monitors have pixels with varying pixel densities. The number of pixels in one point varies
with pixel density. Hence the need for a correction based on the dpi of the monitor.

API constraints:
- While QT's configuration can be obtained in points or pixels, coin3D sets the font size in points.

Solution:
- Continue relying on the local font setting from preferences for coin3d font, albeit by converting from pixels to points.
- Introduce a sketcher wide 3D view scaling factor, as per #4155. This factor is however used for geometry, not for fonts.
- Geometry is scaled to compensate for the scaling factor and the monitor pixel density (the scaling factor is the product
of both scaling factors).
- Derive the 3D view icon size to be 80% (hardcoded) of the 3D view font size. Having constraint icons proportional to contraint label
font size gives consistency to the interface, as constraint icons also have subindices. I do not think it is worth to provide this 80%
as a configurable parameter
- The constraint icon subindex, being a special case of font relative to the accompanying icon, is set to be the 80% of the
icon size (hardcoded). I think it is not worth to provide this as a configurable parameter.

Bonus:
- ViewProviderSketch implements an observer of parameter group and tracks view scaling factor parameter and marker size.
- On change of parameter the inventor nodes are updated and the 3D view redrawn.
- Size information is moved to edit structure for consistency with Marker size.

src/Mod/Sketcher/Gui/SoDatumLabel.cpp

@@ -503,7 +503,7 @@ void SoDatumLabel::GLRender(SoGLRenderAction * action)
     const SbColor& t = textColor.getValue();
 
     // Set GL Properties
-    glLineWidth(2.f);
+    glLineWidth(this->lineWidth.getValue());
     glColor3f(t[0], t[1], t[2]);
 
     if (this->datumtype.getValue() == DISTANCE ||