preferencesdialog: New combobox to change application language

The behavior remains basically as before: when first launched
the application will try to load a translation for the user's
locale, and if one cannot be found it defaults to English.
The difference is that now this is remembered so that upon
further launches the program will go straight to the matching
locale.

See issue #182.

src/PreferencesDialog.h

@@ -38,6 +38,8 @@ private:
     // This works similar to getSettingsValue but returns the default value instead of the value set by the user
     static QVariant getSettingsDefaultValue(const QString& group, const QString& name);
 
+    void fillLanguageBox();
+
     // Cache for storing the settings to avoid repeatedly reading the settings file all the time
     static QHash<QString, QVariant> m_hCache;
 };