Let tray image blend into macOS menu bar

The expected style for macOS menu bar images is a template image than
can adapt to the user's appearance settings. Also, high-res screens
("Retina" in Apple speak) need higher resolution images.

Based on /res/barrier.svg, I created black and transparent SVGs for the
three icon states. I had to remove some details to make the image scale
down to icon size without becoming too crowded, and I drew a new
checkmark and the transfer arrow.

src/gui/res/Barrier.qrc

@@ -54,5 +54,8 @@
         <file>image/spinning-wheel.gif</file>
         <file>icons/16x16/padlock.png</file>
         <file>icons/16x16/barrier-transfering.png</file>
+        <file>icons/32x32/barrier-connected-mask.png</file>
+        <file>icons/32x32/barrier-disconnected-mask.png</file>
+        <file>icons/32x32/barrier-transfering-mask.png</file>
     </qresource>
 </RCC>

src/gui/src/MainWindow.cpp

@@ -67,10 +67,17 @@ static const QString barrierConfigFilter(QObject::tr("Barrier Configurations (*.
 
 static const char* barrierIconFiles[] =
 {
+#if defined(Q_OS_MAC)
+    ":/res/icons/32x32/barrier-disconnected-mask.png",
+    ":/res/icons/32x32/barrier-disconnected-mask.png",
+    ":/res/icons/32x32/barrier-connected-mask.png",
+    ":/res/icons/32x32/barrier-transfering-mask.png"
+#else
     ":/res/icons/16x16/barrier-disconnected.png",
     ":/res/icons/16x16/barrier-disconnected.png",
     ":/res/icons/16x16/barrier-connected.png",
     ":/res/icons/16x16/barrier-transfering.png"
+#endif
 };
 
 static const char* barrierLargeIcon = ":/res/icons/256x256/barrier.ico";
@@ -288,8 +295,13 @@ void MainWindow::saveSettings()
 
 void MainWindow::setIcon(qBarrierState state)
 {
-    if (m_pTrayIcon)
-        m_pTrayIcon->setIcon(QIcon(barrierIconFiles[state]));
+    if (m_pTrayIcon) {
+        QIcon icon = QIcon(barrierIconFiles[state]);
+#if defined(Q_OS_MAC)
+        icon.setIsMask(true);
+#endif
+        m_pTrayIcon->setIcon(icon);
+    }
 }
 
 void MainWindow::trayActivated(QSystemTrayIcon::ActivationReason reason)