calc.el (calc-trail-mode,calc-refresh): Use `face' property to italicize headers.

calc.el (calc-trail-mode,calc-refresh): Use `face' property to italicize headers.
(calc-highlight-selections-with-faces): New variable.
(calc-selected-face, calc-nonselected-face): New faces.

calccomp.el (math-comp-highlight-string): Use
`calc-highlight-selections-with-faces' to determine how to highlight
subformulas.

calc-sel.el (calc-show-selections): Change message to something
appropriate.

calc.texi (Making Selections, Selecting Subformulas)
(Customizing Calc): Mention how to use faces to emphasize selected
subformulas.