Sketcher: Handling of ESC keypress during handler execution

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

Instead of terminating the handler, as mandated before this PR by ViewProviderSketch, ViewProviderSketch
delegates the action to DrawSketchHandler.

DrawSketchHandler implements by default this terminating behaviour, but allows to override it.

DrawSketchDefaultHandler (and all tools deriving from it) implement as default behaviour to cancel if
in initial state, otherwise to reset.

src/Mod/Sketcher/Gui/DrawSketchHandler.cpp

@@ -26,6 +26,8 @@
 
 #include <QGuiApplication>
 #include <QPainter>
+
+#include <Inventor/events/SoKeyboardEvent.h>
 #endif  // #ifndef _PreComp_
 
 #include <Base/Console.h>
@@ -367,6 +369,15 @@ void DrawSketchHandler::toolWidgetChanged(QWidget* newwidget)
     onWidgetChanged();
 }
 
+void DrawSketchHandler::registerPressedKey(bool pressed, int key)
+{
+    // the default behaviour is to quit - specific handler categories may
+    // override this behaviour, for example to implement a continuous mode
+    if (key == SoKeyboardEvent::ESCAPE && !pressed) {
+        quit();
+    }
+}
+
 //**************************************************************************
 // Helpers