chore(macOS): change close-window behavior

On macOS, now closing the window will actually hide the window
instead of quit the application, thus it will match to some other
macOS application's behavior.

Additionally, since our binary is not signed, this could avoid
seeing the request permission dialogs everytime user attempts to
open an image file inside a private place (e.g. places like the
"Downloads" folder and "access data from other apps").

app/main.cpp

@@ -63,7 +63,12 @@ int main(int argc, char *argv[])
     FileOpenEventHandler * fileOpenEventHandler = new FileOpenEventHandler(&a);
     a.installEventFilter(fileOpenEventHandler);
     a.connect(fileOpenEventHandler, &FileOpenEventHandler::fileOpen, [&w](const QUrl & url){
-        if (w.isHidden()) w.showNormal();
+        if (w.isHidden()) {
+            w.setWindowOpacity(1);
+            w.showNormal();
+        } else {
+            w.activateWindow();
+        }
         w.showUrls({url});
         w.initWindowSize();
     });

app/mainwindow.cpp

@@ -71,7 +71,11 @@ MainWindow::MainWindow(QWidget *parent)
     m_exitAnimationGroup->addAnimation(m_fadeOutAnimation);
     m_exitAnimationGroup->addAnimation(m_floatUpAnimation);
     connect(m_exitAnimationGroup, &QParallelAnimationGroup::finished,
+#ifdef Q_OS_MAC
+            this, &QWidget::hide);
+#else
             this, &QWidget::close);
+#endif
 
     GraphicsScene * scene = new GraphicsScene(this);