Make the file browser open files more consistently (bug #45844) * files-dock-widget.cc (files_dock_widget::display_directory): open files in default app except for user defined extensions. Default: m,c,cc,cpp,h,txt * files-dock-widget.cc (files_dock_widget::contextmenu_requested): add action for opening files in the editor * files-dock-widget.cc (files_dock_widget::contextmenu_requested): remove "open in default app" menu and add "open in sytem file explorer" for dirs only. * files-dock-widget.cc (files_dock_widget::contextmenu_open): make use of display_directory for concistency * files-dock-widget.cc (files_dock_widget::contextmenu_open_in_editor): new slot to force opening a file in the editor * files-dock-widget.h: declare new slot "contextmenu_open_in_editor" * settings-dialog.ui: add new lineEdit and label for file extensions setting * settings-dialog.cc (settings_dialog::settings_dialog,::~settings_dialog): read/write new setting "filesdockwidget/txt_file_extensions". Default: m,c,cc,cpp,h,txt

7f19e7f2afa2b1476983003f9cd6df2f5b2a4b08 gnulib-hg