Add FACE to text between START and END.
If START is nil or START to END is empty, add FACE to next typed character
instead. For each section of that region that has a different face property,
FACE will be consed onto it, and other faces that are completely hidden by
that will be removed from the list.
If `facemenu-add-face-function' and maybe `facemenu-end-add-face' are non-nil,
they are used to set the face information.
As a special case, if FACE is `default', then the region is left with NO face
text property. Otherwise, selecting the default face would not have any
effect. See `facemenu-remove-face-function'.