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/widget/Spacer.zig

@@ -6,7 +6,7 @@ const Error = @import("../event.zig").Error;
 
 const log = std.log.scoped(.widget_spacer);
 
-pub fn Widget(comptime Event: type) type {
+pub fn Widget(comptime Event: type, comptime Renderer: type) type {
     if (!isTaggedUnion(Event)) {
         @compileError("Provided user event `Event` for `Layout(comptime Event: type)` is not of type `union(enum)`.");
     }
@@ -25,9 +25,10 @@ pub fn Widget(comptime Event: type) type {
             return null;
         }
 
-        pub fn content(this: *@This()) ![]u8 {
+        pub fn render(this: *@This(), renderer: Renderer) !void {
+            // FIX: this should rather clear the `terminal.Size` provided through the event system using the _renderer_.
             _ = this;
-            return &[0]u8{};
+            _ = renderer;
         }
     };
 }