widgets: Allow widgets to fill remaining space

This is implemented by propagating the expected remaining space to all
widget draw calls. It is not currently as smart as it could be, but it
works well enough to let the last widget in a row/column fill things.

BarWidget-type widgets and the Launcher widget now fill remaining space
if their length is set to 0.

src/widgets/clock.rs

@@ -52,6 +52,7 @@ impl<'a> Widget for Clock<'a> {
         &mut self,
         ctx: &mut DrawContext,
         pos: (u32, u32),
+        _expansion: (u32, u32),
     ) -> Result<DrawReport, ::std::io::Error> {
         let (width, height) = self.size();
         if !(ctx.time.date() != self.cur_time.date()