make octave lexer the default and dynamically set margin width for line numbers * file-editor-tab.cc(update_lexer): octave lexer is the default lexer, remove check for a lexer before setting its apis, reset the width of the margin displaying the line numbers because char size may have changed, (notice_settings): call update_lexer only if there is a file name to prevent the function being called just after tab creation before the file is loaded, do not explicitly use the font metrics for the margin width since they are already respected by setMarginWidth, (dis)connect new slot auto_margin_width to signal linesChanged in order to atuomatically set width for line numbers, (auto_margin_width): set width of margin ahich displays the line numbers depending on max. line number * file-editor-tab.h: new slot auto_margin_width