Correct saving files to DropBox/Drive/OneDrive

* Replaces QSaveFile with QTemporaryFile
* Added backup before save config setting
* This method may cause data loss (see comments)

src/core/Config.cpp

@@ -114,6 +114,7 @@ void Config::init(const QString& fileName)
     m_defaults.insert("AutoSaveAfterEveryChange", false);
     m_defaults.insert("AutoReloadOnChange", true);
     m_defaults.insert("AutoSaveOnExit", false);
+    m_defaults.insert("BackupBeforeSave", false);
     m_defaults.insert("SearchLimitGroup", false);
     m_defaults.insert("MinimizeOnCopy", false);
     m_defaults.insert("UseGroupIconOnEntryCreation", false);