make a floating widget behave like a normal window (bug #37190) * main-window.cc: save and recover the floating- and visible-state of each dock-widget in write_settings() and read_settings(); change window-flag to window if widget is floating at start-up * documentation-dockwidget.cc/.h, files-dockwidget.cc/.h, history-dockwodget.cc/.h, file-editor.cc/.h, terminal-dockwidget.cc/.h, workspace-view.cc/.h: implement slot for signal topLevelChanged where window-flag is changed from widget into window when widget is floating; change icon of all widgets to octave logo

--- a/libgui/src/documentation-dockwidget.h
+++ b/libgui/src/documentation-dockwidget.h
@@ -36,6 +36,8 @@
 public slots:
   /** Slot to steer changing visibility from outside. */
   void handle_visibility_changed (bool visible);
+  /** Slot when floating property changes */
+  void top_level_changed (bool floating);
 
 signals:
   /** Custom signal that tells if a user has clicked away that dock widget. */