gladevcp - offsetpage_widget - bug in mark active solved

- setting one of the rows 1 to 3 with state visible = False, resulted
  in wrong highlighting of the active system in mark_active
- added a signal "selection_changed" to signal GUI the change. (will
  use that signal to set the tool touch off button not sensitive
  if the user select G92 or other non allowed systems
- cleared the code to fit better PEP8 style