refactor project structure (start of redoing atomic branch without atomics)

src/widget/mask.rs

@@ -0,0 +1,20 @@
+use crate::prelude::*;
+
+pub struct Masked {
+    pub inner: WidgetId,
+}
+
+impl Widget for Masked {
+    fn draw(&mut self, painter: &mut Painter) {
+        painter.set_mask(painter.region());
+        painter.widget(&self.inner);
+    }
+
+    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
+        ctx.width(&self.inner)
+    }
+
+    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
+        ctx.height(&self.inner)
+    }
+}