Dynamic widget configuration and layout

1. Implement new layout API, replacing previous hard-coded layout.
2. Make the widgets (previously modules) configurable.
3. Split some widgets into smaller modules.
4. Configuration file (reluctantly JSON at time of writing).
5. More correct damage handling, copying damage to next buffer.

src/buffer.rs

@@ -38,13 +38,43 @@ impl<'a> Buffer<'a> {
         }
     }
 
+    pub fn copy_to(&self, other: &mut Buffer, (x, y, width, height): (i32, i32, i32, i32)) {
+        debug_assert!(self.dimensions == other.dimensions);
+        debug_assert!(self.subdimensions.is_none() && other.subdimensions.is_none());
+
+        if x == 0 && width as u32 == self.dimensions.0 {
+            // Full-width copy
+            let offset = y as isize * self.dimensions.0 as isize;
+            let n = height as usize * self.dimensions.0 as usize;
+            unsafe {
+                std::ptr::copy(
+                    (self.buf.as_ptr() as *const u32).offset(offset),
+                    (other.buf.as_mut_ptr() as *mut u32).offset(offset),
+                    n,
+                );
+            }
+        } else {
+            // Row-by-row copy
+            for cur_y in y as isize..(y + height) as isize {
+                let offset = x as isize + cur_y as isize * self.dimensions.0 as isize;
+                unsafe {
+                    std::ptr::copy(
+                        (self.buf.as_ptr() as *const u32).offset(offset),
+                        (other.buf.as_mut_ptr() as *mut u32).offset(offset),
+                        width as usize,
+                    );
+                }
+            }
+        }
+    }
+
     pub fn subdimensions(
         &mut self,
         subdimensions: (u32, u32, u32, u32),
     ) -> Result<Buffer, ::std::io::Error> {
         let bounds = self.get_bounds();
-        if subdimensions.0 + subdimensions.2 > bounds.2
-            || subdimensions.1 + subdimensions.3 > bounds.3
+        if subdimensions.0 + subdimensions.2 >= bounds.2
+            || subdimensions.1 + subdimensions.3 >= bounds.3
         {
             return Err(::std::io::Error::new(
                 ::std::io::ErrorKind::Other,
@@ -67,14 +97,9 @@ impl<'a> Buffer<'a> {
         })
     }
 
-    pub fn offset(
-        &mut self,
-        offset: (u32, u32),
-    ) -> Result<Buffer, ::std::io::Error> {
+    pub fn offset(&mut self, offset: (u32, u32)) -> Result<Buffer, ::std::io::Error> {
         let bounds = self.get_bounds();
-        if offset.0 > bounds.2
-            || offset.1 > bounds.3
-        {
+        if offset.0 > bounds.2 || offset.1 > bounds.3 {
             return Err(::std::io::Error::new(
                 ::std::io::ErrorKind::Other,
                 format!(
@@ -119,7 +144,7 @@ impl<'a> Buffer<'a> {
 
     pub fn put(&mut self, pos: (u32, u32), c: &Color) -> Result<(), ::std::io::Error> {
         let true_pos = if let Some(subdim) = self.subdimensions {
-            if pos.0 > subdim.2 || pos.1 > subdim.3 {
+            if pos.0 >= subdim.2 || pos.1 >= subdim.3 {
                 return Err(::std::io::Error::new(
                     ::std::io::ErrorKind::Other,
                     format!(