MenuDisableCol optionally hide txt in disabled cell

Fixes #60.

Add optional behavior to hide text in disabled cells by changing text
color to match the disabled background color.

Also added framework for other behaviors via DisableModes.ctl and
HandleDisable.vi:
 -  DisableModes.ctl is a enum typedef where you can define the different
options for disabling behavior for MenuDisableCol.
 -  HandleDisable.vi is called by the class to implement those options,
    given future programmers one location to change.

NOTE:
  I can't imagine why you'd want to have MenuDisableCol disabling a
  SingleValue-type column, but if you do they interact poorly.  Because
  single-value columns essentially have NOPs for both Enable and Disable
  methods, MenuDisableCol could change the text color to hide the text,
  but it would never get changed back.  This would appear to the user that
  cell entries were irretrievably dissappearing.