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/float.rs

@@ -4,7 +4,7 @@ use crate::{AsyncWidget, Frame, Pos, Size, Widget};
 
 #[derive(Debug, Clone, Copy)]
 pub struct Float<I> {
-    inner: I,
+    pub inner: I,
     horizontal: Option<f32>,
     vertical: Option<f32>,
 }
@@ -18,49 +18,68 @@ impl<I> Float<I> {
         }
     }
 
-    pub fn horizontal(mut self, position: f32) -> Self {
-        assert!((0.0..=1.0).contains(&position));
-        self.horizontal = Some(position);
+    pub fn horizontal(&self) -> Option<f32> {
+        self.horizontal
+    }
+
+    pub fn set_horizontal(&mut self, position: Option<f32>) {
+        if let Some(position) = position {
+            assert!((0.0..=1.0).contains(&position));
+        }
+        self.horizontal = position;
+    }
+
+    pub fn vertical(&self) -> Option<f32> {
+        self.vertical
+    }
+
+    pub fn set_vertical(&mut self, position: Option<f32>) {
+        if let Some(position) = position {
+            assert!((0.0..=1.0).contains(&position));
+        }
+        self.vertical = position;
+    }
+
+    pub fn with_horizontal(mut self, position: f32) -> Self {
+        self.set_horizontal(Some(position));
         self
     }
 
-    pub fn vertical(mut self, position: f32) -> Self {
-        assert!((0.0..=1.0).contains(&position));
-        self.vertical = Some(position);
+    pub fn with_vertical(mut self, position: f32) -> Self {
+        self.set_vertical(Some(position));
         self
     }
 
-    pub fn all(self, position: f32) -> Self {
-        assert!((0.0..=1.0).contains(&position));
-        self.horizontal(position).vertical(position)
+    pub fn with_all(self, position: f32) -> Self {
+        self.with_horizontal(position).with_vertical(position)
     }
 
-    pub fn left(self) -> Self {
-        self.horizontal(0.0)
+    pub fn with_left(self) -> Self {
+        self.with_horizontal(0.0)
     }
 
-    pub fn right(self) -> Self {
-        self.horizontal(1.0)
+    pub fn with_right(self) -> Self {
+        self.with_horizontal(1.0)
     }
 
-    pub fn top(self) -> Self {
-        self.vertical(0.0)
+    pub fn with_top(self) -> Self {
+        self.with_vertical(0.0)
     }
 
-    pub fn bottom(self) -> Self {
-        self.vertical(1.0)
+    pub fn with_bottom(self) -> Self {
+        self.with_vertical(1.0)
     }
 
-    pub fn center_h(self) -> Self {
-        self.horizontal(0.5)
+    pub fn with_center_h(self) -> Self {
+        self.with_horizontal(0.5)
     }
 
-    pub fn center_v(self) -> Self {
-        self.vertical(0.5)
+    pub fn with_center_v(self) -> Self {
+        self.with_vertical(0.5)
     }
 
-    pub fn center(self) -> Self {
-        self.all(0.5)
+    pub fn with_center(self) -> Self {
+        self.with_all(0.5)
     }
 
     fn push_inner(&self, frame: &mut Frame, size: Size, mut inner_size: Size) {