Fixes ratsnest crashing on zones with negative net codes.
Pcbnew: fix global deletion bug and minor dialog changes. (fixes lp:1263795)

* Handle different item global deletions correctly.  No more removing of text
  if graphics is selected.
* Layer settings are always obeyed for modules, zones, text, and graphics.
* Text will be removed from copper layers if all layers are selected or
  according to the current layer.
* Added check boxes for "normal" and "locked" footprints to be consistent
  with the naming for the track deletion options.
Pcbnew: fixed SIGFPE arithetic exception. (fixes lp:128339, 1285969, and 1286426)