add: layout and widget dynamic dispatch with interface definitions

src/layout/Pane.zig

@@ -0,0 +1,39 @@
+const std = @import("std");
+const lib_event = @import("../event.zig");
+const widget = @import("../widget.zig");
+
+pub fn Layout(comptime E: type) type {
+    if (!lib_event.isTaggedUnion(E)) {
+        @compileError("Provided user event `E` for `Layout(comptime E: type)` is not of type `union(enum)`.");
+    }
+    return struct {
+        pub const Event = lib_event.MergeTaggedUnions(lib_event.BuiltinEvent, E);
+
+        w: widget.Widget(E) = undefined,
+        c: std.ArrayList(u8) = undefined,
+
+        pub fn init(allocator: std.mem.Allocator, w: widget.Widget(E)) @This() {
+            return .{
+                .w = w,
+                .c = std.ArrayList(u8).init(allocator),
+            };
+        }
+
+        pub fn deinit(this: *@This()) void {
+            this.w.deinit();
+            this.c.deinit();
+            this.* = undefined;
+        }
+
+        pub fn handle(this: *@This(), event: Event) Event {
+            return this.w.handle(event);
+        }
+
+        pub fn content(this: *@This()) *std.ArrayList(u8) {
+            const widget_content = this.w.content();
+            this.c.clearRetainingCapacity();
+            this.c.appendSlice(widget_content.items) catch @panic("OOM");
+            return &this.c;
+        }
+    };
+}