GUI: new setting to restore tabs from previous session; allow silent load file * settings-dialog.ui: new settings check box for previous session restore * settings-dialog.cc (settings_dialog::settings_dialog): load/write new settings * file-editor.h (file-editor::request_open_file): allow silent (no error msg) open file * file_editor_interface.h (file_editor_interface::request_open_file) allow silent (no error msg) open file * file-editor.cc (file_editor::~file_editor): store a list of open tabs in settings (file_editor::request_open_file): allow a silent open file with no error message if file doesn't exist; remove added tab and restore focus, if load fails. (file_editor::contruct): read list of previous tabs from settings (if enabled) * file-editor-tab.h (file-editor-tab:load_file): allow silent load file, return success * file-editor-tab.cc (file-editor-tab:load_file): allow silent load file, return success