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_events.cpp

@@ -151,6 +151,21 @@ void App::display_file(std::string path)
 #endif
 }
 
+void App::share_file(std::string path)
+{
+    if (path.empty())
+        return;
+#ifdef __IOS__
+#elif __OSX__
+    dispatch_async(dispatch_get_main_queue(), ^{
+        [osx_view share_file:[NSString stringWithUTF8String:path.c_str()]];
+    });
+#elif __ANDROID__
+#elif _WIN32
+    // not implemented
+#endif
+}
+
 bool App::mouse_down(int button, float x, float y, float pressure, kEventSource source, bool eraser)
 {
     redraw = true;