Avoid use of deprecated signal QGuiApplication::paletteChanged

Simply use `QEvent::PaletteChange` which is handled in the Qt Widgets GUI
and the Qt Quick GUI anyway. The Plasmoid doesn't need to be changed as it
passes a palette to the icon manager explicitly (to deduce the palette from
the Plasma theme).

syncthingmodel/syncthingicons.cpp

@@ -347,16 +347,6 @@ IconManager::IconManager(const QPalette *palette)
     , m_distinguishTrayIcons(false)
 {
     m_forkAwesomeRenderer.warnIfInvalid();
-#ifdef __GNUC__
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
-#endif
-    if (!palette) {
-        QObject::connect(qGuiApp, &QGuiApplication::paletteChanged, this, &IconManager::setPalette);
-    }
-#ifdef __GNUC__
-#pragma GCC diagnostic pop
-#endif
 }
 
 void IconManager::applySettings(

tray/gui/quick/app.cpp

@@ -549,10 +549,14 @@ bool App::eventFilter(QObject *object, QEvent *event)
     }
     switch (event->type()) {
     case QEvent::ApplicationPaletteChange:
-        if (m_app && m_imageProvider) {
-            m_imageProvider->setDefaultColor(m_app->palette().color(QPalette::Normal, QPalette::Text));
-        }
         qDebug() << "Application palette has changed";
+        if (m_app) {
+            const auto palette = m_app->palette();
+            IconManager::instance(&palette).setPalette(palette);
+            if (m_imageProvider) {
+                m_imageProvider->setDefaultColor(palette.color(QPalette::Normal, QPalette::Text));
+            }
+        }
         applyDarkmodeChange(m_darkColorScheme, QtUtilities::isPaletteDark());
         break;
     default:;

tray/gui/traywidget.cpp

@@ -630,11 +630,14 @@ bool TrayWidget::event(QEvent *event)
 {
     const auto res = QWidget::event(event);
     switch (event->type()) {
-    case QEvent::PaletteChange:
+    case QEvent::PaletteChange: {
+        const auto palette = QPalette();
+        IconManager::instance(&palette).setPalette(palette);
         setBrightColorsOfModelsAccordingToPalette();
         setLabelPixmaps();
         setTrafficPixmaps(true);
         break;
+    }
     case QEvent::LanguageChange:
         m_ui->retranslateUi(this);
         applyLauncherSettings(false, true, false);