make size of toolbar icons depending on scaling of display * files-dock-widget.cc (notice_settings): set icon size to small, normal or large by using qt pixel metrics * file-editor.cc notice_settings): set icon size to small, normal or large by using qt pixel metrics * main-window.cc (notice_settings): set icon size to small, normal or large by using qt pixel metrics * settings-dialog.ui: use radio buttons for small, normal and large icon size instead of spin box with pixel sizes * settings-dialog.cc (settings_dialog): initialize the radio buttons depending on the settings file; (write_changed_settings): save the icon size depending on radio buttons