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/app.zig

@@ -45,9 +45,9 @@ pub fn App(comptime E: type, comptime R: fn (comptime bool) type, comptime fulls
     }
     return struct {
         pub const Event = mergeTaggedUnions(event.SystemEvent, E);
-        pub const Layout = @import("layout.zig").Layout(Event);
-        pub const Widget = @import("widget.zig").Widget(Event);
         pub const Renderer = R(fullscreen);
+        pub const Layout = @import("layout.zig").Layout(Event, Renderer);
+        pub const Widget = @import("widget.zig").Widget(Event);
 
         queue: Queue(Event, 256) = .{},
         thread: ?std.Thread = null,