Lays out a string out in a field N average characters wide and m deep.
m is optional and defaults to 1 but N must be specified The
string is read-only (unlike %rs - Edit String, the string cannot be changed by the user). It
can, however, be changed by the programmer. The change is followed by a call to
window_update@(string) in order to make a change visible. The string is
re-drawn each time the window is renewed. (%`rs provides a similar effect.)
When used as the last item in a status bar, the control will automatically
expand to fill the remaining space.