Keep a set of all the tags from the posts stored

This allows later on to get post-specific tags.

src/model/danboorupostmodel.cpp

@@ -57,6 +57,7 @@ void DanbooruPostModel::addPost(Danbooru::DanbooruPost *post)
 
     beginInsertRows(QModelIndex(), m_items.size(), m_items.size());
     m_items.append(post);
+    m_postTags.unite(post->tags());
     endInsertRows();
 }
 
@@ -147,5 +148,11 @@ QHash< int, QByteArray > DanbooruPostModel::roleNames() const
     return roles;
 }
 
+QStringList DanbooruPostModel::postTags() const
+{
+
+    return m_postTags.toList();
+
+}
 
 };