mod: replace Layout.content with Layout.render

The App.Renderer is used for the new `Layout.render` method. Each layout
renders itself now with corresponding renderers which might only update
parts of the screen, etc.

src/widget.zig

@@ -10,8 +10,11 @@
 //!
 //! Each `Widget` may cache its content and should if the contents will not
 //! change for a long time.
+const std = @import("std");
 const isTaggedUnion = @import("event.zig").isTaggedUnion;
 
+const log = std.log.scoped(.widget);
+
 pub fn Widget(comptime Event: type) type {
     if (!isTaggedUnion(Event)) {
         @compileError("Provided user event `Event` for `Layout(comptime Event: type)` is not of type `union(enum)`.");
@@ -31,6 +34,12 @@ pub fn Widget(comptime Event: type) type {
 
         // Handle the provided `Event` for this `Widget`.
         pub fn handle(this: *WidgetType, event: Event) ?Event {
+            switch (event) {
+                .resize => |size| {
+                    log.debug("received size: .{{ .cols = {d}, .rows = {d} }}", .{ size.cols, size.rows });
+                },
+                else => {},
+            }
             return this.vtable.handle(this, event);
         }