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_cloud.cpp

@@ -146,7 +146,8 @@ void App::cloud_browse()
             m->m_message->set_text("Download in progress");
             async_redraw();
             async_end();
-            download(dialog->selected_path, [this,m](float p){
+            std::string url = "https://panopainter.com/cloud/cloud-dwl.php?file=" + dialog->selected_file;
+            download(url, dialog->selected_path, [this,m](float p){
                 static char progress[256];
                 sprintf(progress, "Download in progress %.2f%%", p * 100.f);
                 async_start();