Use new with_* naming scheme

All builder-like functions are now named with_*. There is also now a way
to set each property imperatively with only a mutable reference.

The only widgets I haven't yet converted to this style are the Join*
widgets; they're a bit harder to figure out an appropriate API for.

src/widgets/layer.rs

@@ -4,8 +4,8 @@ use crate::{AsyncWidget, Frame, Size, Widget};
 
 #[derive(Debug, Clone, Copy)]
 pub struct Layer<I1, I2> {
-    below: I1,
-    above: I2,
+    pub below: I1,
+    pub above: I2,
 }
 
 impl<I1, I2> Layer<I1, I2> {