ask for saving modified editor files if octave is closed (bug #38689) * files-editor-tab.cc(constrctor): init new flag indicating if app is closing, (check_file_modified): message box is modal if app is closing, no cancel, parent of box is the editor's tab widget for a correct palcement of the box, (conditional_close): new second arg: flag for closing app (default false), it is stored in the tab's class wide flag * file-editor-tab.h: second arg for conditional_close and new class wide flag * file-editor.cc(destructor): sending close requests to all editor tabs with flag indicating the application is closing (add_file_editor_tab): new arg for fetab_close_request and conditional_close * file-editor.h: new 2nd arg for fetab_close_request (closing app, def. false) * main-window.cc(destructor): delete editor window first for showing the message boxes for modified editor files in front of a complete gui

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