SETSTATUS expression , expression , expression
    
   Synopsis:
      Updates a 'main' window status bar
    
   Notes:
      Updates a status bar that was added to the 'main' window with an ADDSTATUS
         or ADDCONSTATUS statement.
      The first expression is the status bar's unique number. If the status bar 
         doesn't exist, the SETSTATUS statement is ignored, but the number isn't 
         an integer, 0 or above, you'll see an error message.
      The second and third expressions should evaluate to numbers displayed in
         the status bar. Any decimal number is acceptable, even negative 
         numbers. If these expressions don't evaluate to a number, a question 
         mark is displayed in its place.
      After an ADDSTATUS statement, the first number is expected (but not
         required) to be less than or equal to the second one. After an 
         ADDCONSTATUS statement, the sum of the two numbers is expected (but not
         required) to be constant, at least in the short term.

   Examples:
      REM Character's health points are at 50%
      ADDSTATUS 1, "HP"
      SETSTATUS 1, 50, 100

      REM Character's energy points are very low
      ADDSTATUS 2, "EP"
      SETSTATUS 2, 0.017, 1

      REM Show a status bar where the two numbers are added
      REM Character has 900xp, and needs 100 more to level up
      ADDCONSTATUS 3, "XP"
      SETSTATUS 3, 900, 100
