Gets a window's label. Control windows generally display their
label in some way. Frames and dialogs display their label as a
window title. Panels do not display their label, but the label
can be used for identification purposes. Buttons and check boxes
can have bitmap labels (only when they are created with bitmap
labels), but all other windows have string labels. The label
string may contain ampersands (``&''), which serve as keyboard
navigation annotations for controls under Windows and X. The
ampersands are not part of the displayed label of a control;
instead, ampersands are removed in the displayed label (under
all platforms), and any character preceeding an ampersand is
underlined (Windows and X) indicating that the character is a
mnemonic for the control. Double ampersands are converted into a
single ampersand (with no displayed underline). See also on-
traverse-char. If the window does not have a label, #f is
returned.