changed logic to not check if "linux" but check "not windows" to fix issue on mac (which does not evaluate to "is linux")

Author: Death111

src/main/java/de/doubleslash/keeptime/App.java

@@ -239,7 +239,7 @@ private void initialisePopupUI(final Stage primaryStage) throws IOException {
       final ViewControllerPopup viewControllerPopupController = loader.getController();
       viewControllerPopupController.setStage(popupViewStage);
 
-      if (!OS.isLinux()) {
+      if (OS.isWindows()) {
          globalScreenListener = new GlobalScreenListener();
          globalScreenListener.register(model.useHotkey.get());
          globalScreenListener.setViewController(viewControllerPopupController);

src/main/java/de/doubleslash/keeptime/view/SettingsController.java

@@ -183,7 +183,7 @@ private void initialize() {
       LOG.debug("load substages");
       LOG.debug("set version label text");
 
-      if (OS.isLinux()) {
+      if (!OS.isWindows()) {
          LOG.info("Disabling unsupported settings for Linux.");
          useHotkeyCheckBox.setDisable(true);
          hotkeyLabel.setDisable(true);
@@ -208,7 +208,7 @@ private void initialize() {
       saveButton.setOnAction(ae -> {
          LOG.info("Save clicked");
 
-         if (OS.isLinux()) {
+         if (!OS.isWindows()) {
             if (hoverBackgroundColor.getValue().getOpacity() < 0.5) {
                hoverBackgroundColor.setValue(Color.rgb((int) (hoverBackgroundColor.getValue().getRed() * 255),
                      (int) (hoverBackgroundColor.getValue().getGreen() * 255),