Define the LNVL.Scene.Pause constant

We need a more readable way to have pauses in scripts than by writing
empty strings.  So it seems reasonable to have a constant for scenes
that translates into the ‘no-op’ opcode, accomplishing the same amount
of nothing that writing an empty string in a script would accomplish.
Use LNVL.Scene.Pause instead of empty strings in the ‘11’ example

Not only does this serve as a test of the constant, to make sure the
engine treats it as the correct opcode, but it hopefully shows that
scripts can be more readable by using this constant.
Document LNVL.Scene.Pause in the Howto
Merge branch 'pauses-in-scenes'

* pauses-in-scenes:
  Document LNVL.Scene.Pause in the Howto
  Use LNVL.Scene.Pause instead of empty strings in the ‘11’ example
  Define the LNVL.Scene.Pause constant