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

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