general cleanup

-use of the override keyword
-delete unused code
-const correctness
-more uniform code style
-for each with const references when possible
-getters no longer use the word 'get'
-others

src/utils/systemnotification.cpp

@@ -18,7 +18,7 @@ void SystemNotification::sendMessage(
         const QString &title,
         const int timeout)
 {
-    if(!ConfigHandler().getDesktopNotification()) {
+    if(!ConfigHandler().desktopNotificationValue()) {
         return;
     }