Gui: refactor SoLinearDragger (#21451)

* Gui: refactor SoLinearDragger

* further improvements

* allow changing label visibility

* change SoLinearDraggerContainer to not inherit from SoDragger

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Co-authored-by: captain <captain@captain.captain.com>

src/Gui/SoFCDB.cpp

@@ -143,7 +143,7 @@ void Gui::SoFCDB::init()
     SoFCTransform                   ::initClass();
     SoAutoZoomTranslation           ::initClass();
     MarkerBitmaps                   ::initClass();
-    SoTransformDragger                 ::initClass();
+    SoTransformDragger              ::initClass();
     SmSwitchboard                   ::initClass();
     SoFCSeparator                   ::initClass();
     SoFCSelectionRoot               ::initClass();