window_decorator ->label: format=char_array*,
argument=any ...Define a window-level (opposed to frame<->label)
label for the window. The label is visualised by window_decorator<-label_text
using the font pce.WindowDecorator.label_font.
If the format is a markup
object, the label shows its markup and the arguments are escaped (see
class markup).
Otherwise the formatted text is shown as is. A label that does not fit
the width of the window ends in ....
window_decorator<-label
returns the markup if the label is a markup and otherwise its text. It
fails if window_decorator<-label_text
yields @nil.
