Format a string out of a mode line format specification.
First arg FORMAT specifies the mode line format (see `mode-line-format'
for details) to use.
By default, the format is evaluated for the currently selected window.
Optional second arg FACE specifies the face property to put on all
characters for which no face is specified. The value nil means the
default face. The value t means whatever face the window's mode line
currently uses (either `mode-line' or `mode-line-inactive',
depending on whether the window is the selected window or not).
An integer value means the value string has no text
Optional third and fourth args WINDOW and BUFFER specify the window
and buffer to use as the context for the formatting (defaults
are the selected window and the WINDOW's buffer). (fn FORMAT &optional FACE WINDOW BUFFER)