update internal __mfile_encoding__ when gui default encoding changes (bug #54310) * gui-preferences.h: define constants for editor default encoding prefs * main-window.cc (main_window): initialize new class variable storing the default encoding; (notice_settings): check for changes in default encoding and update __mfile_encoding__ via worker thread if required; (gui_preference_adjust): use constants for pref key; * main-window.h: new class variable storing the default encoding * resource-manager.cc: include gui-preferences.h; (do_combo_encoding): use constants for key and default of encoding

--- a/libgui/src/main-window.h	Sun Jul 29 11:30:33 2018 +0200
+++ b/libgui/src/main-window.h	Sun Jul 29 12:57:07 2018 +0200
@@ -365,6 +365,8 @@
 
     QHash<QMenu*, QStringList> m_hash_menu_text;
 
+    QString m_default_encoding;
+
     //! Toolbar.
 
     QStatusBar *m_status_bar;