Apply Qt settings during runtime

It is no longer required to restart the application to change settings like
the widget style and palette.

README.md

@@ -571,8 +571,9 @@ tweaking:
   WindowsArguments = darkmode=1
   ```
 
-After changing these settings, restart Syncthing Tray. It is not sufficient to
-close the last Windows; the process needs to be restarted.
+When using Syncthing Tray 1.3.x or older, you need to restart Syncthing Tray for
+these changes to have any effect. It is not sufficient to close the last window;
+the process needs to be restarted.
 
 Note that one can alternatively also enable Windows' "High contrast" setting which
 seems to bring back the traditional theming/coloring (which has normally been

tray/application/main.cpp

@@ -220,6 +220,7 @@ int runApplication(int argc, const char *const *argv)
         QObject::connect(&singleInstance, &SingleInstance::newInstance, &runApplication);
         Settings::restore();
         auto &settings = Settings::values();
+        settings.qt.disableNotices();
         settings.qt.apply();
         qtConfigArgs.applySettings(true);
         if (assumeFirstLaunchArg.isPresent()) {

tray/gui/traywidget.cpp

@@ -631,6 +631,7 @@ bool TrayWidget::event(QEvent *event)
 
 void TrayWidget::applySettingsOnAllInstances()
 {
+    Settings::values().qt.apply();
     for (TrayWidget *instance : s_instances) {
         instance->applySettings();
     }