Handle no System Tray on Linux (Gnome)

- What
  - On Linux
    - Show Configure Screen, even if not first run experience
    - Do no show system tray on Linux
    - Quit app on closing Configure Screen
  - On Windows, Mac
    - Show Configure screen only if first run experience
    - Show system tray always
    - Do not quit app on closing Configure Screen

- Why
  - Configure screen is the only GUI element on Linux. So closing it
    should close the application
  - On Windows, Mac the system tray exists, so app should not be closed
    on closing configure screen

src/main.py

@@ -1,4 +1,5 @@
 # Standard Packages
+from platform import system
 import signal
 import sys
 
@@ -37,8 +38,14 @@ def run():
     else:
         # Setup GUI
         gui = QtWidgets.QApplication([])
-        gui.setQuitOnLastWindowClosed(False)
         configure_screen = ConfigureScreen(args.config_file)
+
+        # System tray is only available on Windows, MacOS.
+        # On Linux (Gnome) the System tray is not supported.
+        # Since only the Configure Window is available
+        # Quitting it should quit the application
+        if system() in ['Windows', 'Darwin']:
+            gui.setQuitOnLastWindowClosed(False)
             tray = create_system_tray(gui, configure_screen)
             tray.show()
 
@@ -46,8 +53,8 @@ def run():
         configure_server(args, required=False)
         server = ServerThread(app, args.host, args.port, args.socket)
 
-        # Trigger First Run Experience, if required
-        if args.config is None:
+        # Show Configure Screen on Linux (etc.) or First Run Experience
+        if args.config is None or system() not in ['Windows', 'Darwin']:
             configure_screen.show()
 
         # Setup Signal Handlers