1st installment of gizmos refactoring

1) GLCanvas3D::Gizmos moved into a standalone class GLGizmosManager

2) GLGizmosManager handles mouse events by calling its on_mouse method (wip)

src/slic3r/GUI/GLToolbar.hpp

@@ -239,6 +239,8 @@ private:
         bool right;
         GLCanvas3D* parent;
 
+        MouseCapture() { reset(); }
+
         bool any() const { return left || middle || right; }
         void reset() { left = middle = right = false; parent = nullptr; }
     };