refactor: Move LineTool event handling to LineTool and add tool APIs

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/ViewportWidget.h

@@ -53,6 +53,9 @@ public:
     bool isSnappingHorizontal() const { return m_isSnappingHorizontal; }
     bool isSnappingVertical() const { return m_isSnappingVertical; }
 
+    void addLine(const gp_Pnt& start, const gp_Pnt& end);
+    void deactivateActiveTool();
+
 public slots:
     void onSketchModeStarted(SketchPlane plane);
     void onSketchModeEnded();