Better emoji picker (#73)

Adds a better, custom, emoji picker.

Reviewed-on: #73

lib/models/emoji_category.dart

@@ -0,0 +1,12 @@
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/emoji.dart";
+import "package:flutter/widgets.dart";
+part "emoji_category.freezed.dart";
+
+@Freezed(toJson: false, fromJson: false)
+class EmojiCategory({
+  required final Widget icon,
+  required final String name,
+  required final IList<Emoji> emojis,
+}) with _$EmojiCategory;