(%-constructs are allowed when the string is the entire mode-line-format
or when it is found in a cons-cell or a list)
%b -- print buffer name. %f -- print visited file name.
- %F -- print frame name.
+ %F -- print frame name. %T -- print frame name if on tty.
%* -- print %, * or hyphen. %+ -- print *, % or hyphen.
%& is like %*, but ignore read-only-ness.
% means buffer is read-only and * means it is modified.