add export layers, export png, import brush padding

src/app.h

@@ -57,6 +57,7 @@ public:
     NodeMessageBox* msgbox;
     NodeSettings* settings;
     NodePopupMenu* popup = nullptr;
+    NodePopupMenu* subpopup = nullptr;
     NodePopupMenu* menu_file = nullptr;
     NodePopupMenu* menu_edit = nullptr;
     NodePopupMenu* menu_layers = nullptr;
@@ -178,7 +179,8 @@ public:
     void dialog_save_ver();
     void dialog_open();
     void dialog_browse();
-	void dialog_export();
+    void dialog_export(std::string ext);
+    void dialog_export_layers();
 	void dialog_export_cubes();
     void dialog_layer_rename();
     void dialog_resize();