Sketcher: Grid - correction of design decisions

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

Removal of ViewProviderGridExtension properties:
-GridStyle
-GridSnap

Improvement of ViewProviderGridExtension API:
- color via App::Color

Toolbar grid command:
- Conversion of Snapping to edit parameter
- Snap gets a class enum to support future Snapping functionalities as edit parameter

Sketcher Settings:
- Removal of snap preference

Behaviour:
- ShowGrid, GridAuto, GridSize preferences do not change any existing VP. They apply to any newly created sketch.
- Do not limit grid to hardcoded values

Snap to grid:
- Code updated to use closestpoint functionality provided by the extension,

Several other fixes

src/Mod/Sketcher/Gui/SketcherSettings.cpp

@@ -149,7 +149,6 @@ void SketcherSettingsGrid::saveSettings()
 {
     ui->checkBoxShowGrid->onSave();
     ui->gridSize->onSave();
-    ui->checkBoxGridSnap->onSave();
     ui->checkBoxGridAuto->onSave();
     ui->gridSizePixelThreshold->onSave();
     ui->gridLineColor->onSave();
@@ -172,7 +171,6 @@ void SketcherSettingsGrid::loadSettings()
 {
     ui->checkBoxShowGrid->onRestore();
     ui->gridSize->onRestore();
-    ui->checkBoxGridSnap->onRestore();
     ui->checkBoxGridAuto->onRestore();
     ui->gridSizePixelThreshold->onRestore();
     ui->gridLineColor->onRestore();