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/empty.rs

@@ -4,7 +4,7 @@ use crate::{AsyncWidget, Frame, Size, Widget};
 
 #[derive(Debug, Default, Clone, Copy)]
 pub struct Empty {
-    size: Size,
+    pub size: Size,
 }
 
 impl Empty {
@@ -12,17 +12,17 @@ impl Empty {
         Self { size: Size::ZERO }
     }
 
-    pub fn width(mut self, width: u16) -> Self {
+    pub fn with_width(mut self, width: u16) -> Self {
         self.size.width = width;
         self
     }
 
-    pub fn height(mut self, height: u16) -> Self {
+    pub fn with_height(mut self, height: u16) -> Self {
         self.size.height = height;
         self
     }
 
-    pub fn size(mut self, size: Size) -> Self {
+    pub fn with_size(mut self, size: Size) -> Self {
         self.size = size;
         self
     }