Request automatic widget length

Previously widget length was set to panel size which resulted in a lot of forced, unconfigurable empty space around progress bar.

The thickness of progress bar and amount of empty space around it can be configured in gtk.css. So there is no need to configure it from plugin settings.

It would be nice to include details how to do configuration though. But I'm not sure where it should go and whether https://docs.xfce.org/panel-plugins/xfce4-timer-plugin is an appropriate place.

Sample configuration:

#xfce4-timer-plugin-16 progressbar.vertical trough,
#xfce4-timer-plugin-16 progressbar.vertical progress {
  min-width: 8px;
  margin-left: 4px;
  margin-right: 4px;
}
#xfce4-timer-plugin-16 progressbar.horizontal trough,
#xfce4-timer-plugin-16 progressbar.horizontal progress {
  min-height: 8px;
  margin-top: 4px;
  margin-bottom: 4px;
}