add a scrolling text widget

[dankamongmen]

Add ncscrolltext or something, which has some fixed geometry, and an associated body of text (not necessarily larger than the widget area, but presumably so). If it fits wholly within the widget, lay the text out with ncplane_puttext() for nice word breaking etc. otherwise, scroll the text at some configurable rate.

the implementation is easy. the hard part is driving it in a thread-safe manner, which almost has to be done from the client context. together with how fade() needs be done, maybe we need a ncfoo_offer_clocktick() or something. or maybe not. think this through and come up with a sensible design.

maybe we don't accept the time as a parameter, and just offer the function ncscrolltext_advance(), and the user just has to explicitly call that via whatever timing mechanism they like.