Draft: Resolve "Create game model"

Closes #2 (closed)

core/src/com/wordbattle/game/model/Category.java

+package com.wordbattle.game.model;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Category {
+    private String name;
+    private List<Word> words;
+
+    public Category(String name) {
+        this.name = name;
+        this.words = new ArrayList<>();
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public List<Word> getWords() {
+        return words;
+    }
+
+    public void addWord(Word word) {
+        words.add(word);
+    }
+}