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/widgets/mod.rs

@@ -0,0 +1,8 @@
+pub mod backlight;
+pub mod bar_widget;
+pub mod battery;
+pub mod calendar;
+pub mod clock;
+pub mod date;
+pub mod launcher;
+pub mod sound;