New Navigation & Pathfinding support for 2D

-Added Navigation & NavigationPolygon nodes
-Added corresponding visual editor
-New pathfinding algorithm is modern and fast!
-Similar API to 3D Pathfinding (more coherent)

core/dvector.h

@@ -262,6 +262,23 @@ public:
 			w[bs+i]=r[i];
 	}
 
+
+	Error insert(int p_pos,const T& p_val) {
+
+		int s=size();
+		ERR_FAIL_INDEX_V(p_pos,s+1,ERR_INVALID_PARAMETER);
+		resize(s+1);
+		{
+			Write w = write();
+			for (int i=s;i>p_pos;i--)
+				w[i]=w[i-1];
+			w[p_pos]=p_val;
+		}
+
+		return OK;
+	}
+
+
 	bool is_locked() const { return mem.is_locked(); }
 	
 	inline const T operator[](int p_index) const;