add share button and implement AirDrop share in OSX, update info.plist to set on Mac OS the default app for PPI, on iOS include the Documents/Inbox subfolder used to store the received files from AirDrop

src/app_layout.cpp

@@ -420,6 +420,12 @@ void App::init_menu_file()
                     popup->mouse_release();
                     popup->destroy();
                 };
+            if (auto b = popup->find<NodeButtonCustom>("file-share"))
+                b->on_click = [this](Node*) {
+                    share_file(doc_path);
+                    popup->mouse_release();
+                    popup->destroy();
+                };
             if (auto b = popup->find<NodeButtonCustom>("file-resize"))
                 b->on_click = [this](Node*) {
                     dialog_resize();