Used in determining y for a TreeWindow's ListView.
After calculating for buttons and known gaps and
determining the border width of the internal window,
there's still some missing pixels.
I imagine this can be determined programatically,
but I'm not hitting on it - this'll do for now.
If a text field has no border, the text is placed
lower so that the space occupied by the missing border
is used. If a borderless text field is being used to
label a bordered text field this difference must be
take into account during placement. Hence, LABELBORDERGAP.