Add an item to the tool bar.
ICON names the image, DEF is the key definition and KEY is a symbol
for the fake function key in the menu keymap. Remaining arguments
PROPS are additional items to add to the menu item specification. See
Info node `(elisp)Tool Bar'. Items are added from left to right.
ICON is the base name of a file containing the image to use. The
function will first try to use low-color/ICON.xpm if `display-color-cells'
is less or equal to 256, then ICON.xpm, then ICON.pbm, and finally
ICON.xbm, using `find-image'.
Use this function only to make bindings in the global value of `tool-bar-map'.
To define items in any other map, use `tool-bar-local-item'.