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
Fix #371, mist and flood buttons in AXIS

This reverts part of commit 683566630, which maybe was
added or left in by mistake...?
Fix AXIS's magic disappearing spindle controls

 * This code had not kept up with the proliferation of variants of the
   spindle-speed-out output

 * Previously if you hooked up a spindle-speed, but not forward or
   reverse or on, you'd get only increase and decrease buttons.  I think in
   this case, although it's a weird setup, you should get the full complement.
mux_generic: add missing return value check
Merge remote-tracking branch 'origin/2.6'