Add NavigationServer map_force_update() function

Adds map_force_update() function to NavigationServer. This function immediately flushes the Navigationserver command queue and recalculates all navigationmeshes and region connections for a specific map.

(cherry picked from commit fdea269805)

servers/navigation_2d_server.h

@@ -89,6 +89,8 @@ public:
 	virtual Array map_get_regions(RID p_map) const;
 	virtual Array map_get_agents(RID p_map) const;
 
+	virtual void map_force_update(RID p_map);
+
 	/// Creates a new region.
 	virtual RID region_create() const;