Add MenuCol class

Implements a column type that spawns a pop-up menu when the user clicks
on it.  The menu contents are defined in the private data for the column,
and are populated into the table when the user makes a selection.
Add allowed value checking to MenuCol

If the column does not accept variables as entries, it should only allow
internal contents to contain valid menu entries.  To this end, in both
SetCol.vi and ValChange.vi, if the column does not allow variables, check
contents against list of menu entries.

If not found in the list, present a one-button dialog warning the user
and do not change the value.

If column is a variable _OR_ the entry is valid, make the parent call to
update the table.
Add TestMenu test device (but do not add to list)

Generate a TestMenu device to demo/test the functionality of the MenuCol
column type, but do not add it to the list of devices.
Fix error checking on table entries in MenuCol

Fixed valid entry so now it actually checks only non-variable columns and
reports a problem only if the entry isn't valid.
MenuCol allows double clicks when isVariable=true

Previously all left-clicks routed to the shortcut menu, now a double click
on a cell that allows variables will allow text entry in the cell.