Extensions to Fld* words to work with Fields

: FldDeleteAll ( fldP -- )
Clear the field.

: FldCanAppend ( fldP -- n )
n is the number of characters that can be added to the field.

: FldGoToEnd ( fldP -- )
Set the insertion point after the last character in the field.

1 value FldAppendPreallot
When you use FldAppend to append a string to a target field, but there is 
not enough space in the field for the string, FldAppend deletes a number 
of characters from the beginning of the field. At least FldAppendPreallot 
characters will be deleted.

: MemAddrDelBegin ( n addr u -- )
Delete n bytes from the beginning of the string (addr u ), with shift.

: MemHandleDelBegin ( n h -- )
Delete n bytes from the beginning of the chunk with handle h, with shift 
and resize 

: FldDelBegin ( fldP n -- )
Delete n characters from the beginning of the field, without updating it.

: FldAppend ( fldP addr u -- )
Append the string at (addr u) to the end of the field. If there's no room, 
delete from the beginning of the field at least FldAppendPreallot 
characters. 

: FldGetScrollBar ( fldP -- val min max page )
Get the values of the scrollbar.
