XTREE enhancement: New cformat code P may be used with editable cells (cformat codes E, l) to indicate that the first position (after any cell code bytes such as the color byte used with cformat B) is reserved for either a space (indicating editable) or a "|" indicating that the cell is not editable.
Note that the "|" prefix has long been supported as a code to indicate that the cell is disabled for editing, but it is sometimes awkward to use because of confusion over whether to shift the remaining characters over. The new P option allows you to reserve a position (same concept as with the background cell color byte used with B), which may be easier to use, depending on your application logic.
Also note that feature is not necessary with the shared lists (cformat s used with CellList or CellListX), because in those cases, a space is already reserved for a prefix code, in which "|" is already an option.
