mod: change widget interface Widget.content replaced with Widget.render

The .resize `Event` has been adapted to include an _anchor_, which
provide the full necessary information for each widget where to render
on the screen with what requested size. Each Widget can then dynamically
decide how and what to render (i.e. provide placeholder text in case the
size is too small, etc.).

src/app.zig

@@ -47,7 +47,7 @@ pub fn App(comptime E: type, comptime R: fn (comptime bool) type, comptime fulls
         pub const Event = mergeTaggedUnions(event.SystemEvent, E);
         pub const Renderer = R(fullscreen);
         pub const Layout = @import("layout.zig").Layout(Event, Renderer);
-        pub const Widget = @import("widget.zig").Widget(Event);
+        pub const Widget = @import("widget.zig").Widget(Event, Renderer);
 
         queue: Queue(Event, 256) = .{},
         thread: ?std.Thread = null,