Make rio and libframe play nicer with fonts with fractional
alpha channels, by always using stringbg and not string.
Using plain string will redraw the text on top of what is already
there.  If the text is partially transparent, the underlying text
bleeds through.  Since rio grays out text in non-foreground 
windows, using plain string results in text getting darker and
darker as you switch between text windows.

This attempt certainly fixes bugs present in previous attempts
involving selected text and losing the tick.

I have tested it quite a bit and believe it has a good chance of being correct.