add(element): interface for injecting user behavior to containers

Some additional refactoring and documentation updates have also been
applied.

src/element.zig

@@ -0,0 +1,23 @@
+//! Interface for Element's which describe the contents of a `Container`.
+const Cell = @import("cell.zig");
+const Size = @import("size.zig").Size;
+
+pub fn Element(Event: type) type {
+    return struct {
+        ptr: *anyopaque = undefined,
+        vtable: *const VTable = &.{},
+
+        pub const VTable = struct {
+            handle: ?*const fn (ctx: *anyopaque, event: Event) anyerror!void = null,
+            content: ?*const fn (ctx: *anyopaque, cells: []Cell, size: Size) anyerror!void = null,
+        };
+
+        pub inline fn handle(this: @This(), event: Event) !void {
+            if (this.vtable.handle) |handle_fn| try handle_fn(this.ptr, event);
+        }
+
+        pub inline fn content(this: @This(), cells: []Cell, size: Size) !void {
+            if (this.vtable.content) |content_fn| try content_fn(this.ptr, cells, size);
+        }
+    };
+}