added a valentine's day splash screen image

Conflicts:
	collects/drracket/private/drracket-normal.rkt
removed robby-specific references and commented out the line that clobbers heart.png in the collects directory
fixup the heart bitmap so it has an alpha channel and then add it into the weekend/weekday rotation

Conflicts:
	collects/drracket/private/drracket-normal.rkt
move the mkheart.rkt script into drracket/private

and modernize it slightly.