change gui preferences from within the worker thread (bug #49119) * main-window.cc (construct_octave_qt_link): connect the slot accessing the preferences with the related octave-qt-link signal; (gui_preference): slot for reading the desired preference and for setting a new value if the given value is not empty, finally resuming the worker thread by unlocking a mutex * main-window.h: new slot gui_preference * octave-qt-link.cc (do_gui_preference): new method signaling a request for accessing the preferences to the gui, waiting for the result by a mutex and returning the old value of the preference as a string * octave-qt-link.h: implementation of purely virtual method octave_link::do_gui_preference, new signal gui_preference_signal * octave-link.cc (__octave_link_gui_preference__) new octave internal function for accessing the gui preferences by calling octave_link::gui_preference * octave-link.h: (gui_preference): new static method calling purely virtual method do_gui_preference, which is implemented in octave_qt_link

--- a/libgui/src/octave-qt-link.h	Wed Jul 18 14:23:01 2018 -0400
+++ b/libgui/src/octave-qt-link.h	Wed Jul 18 20:54:05 2018 +0200
@@ -138,6 +138,8 @@
 
     void do_show_preferences (void);
 
+    std::string do_gui_preference (const std::string& key,
+                                   const std::string& value);
     void do_show_doc (const std::string& file);
     void do_register_doc (const std::string& file);
     void do_unregister_doc (const std::string& file);
@@ -193,6 +195,9 @@
 
     void show_preferences_signal (void);
 
+    void gui_preference_signal (const QString&, const QString&,
+                                QMutex*, QString*);
+
     void show_doc_signal (const QString& file);
 
     void register_doc_signal (const QString& file);