Emoji picker: Implement cell and header view.

Riot/Modules/Room/EmojiPicker/EmojiPickerHeaderView.swift

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+/*
+ Copyright 2019 New Vector Ltd
+ 
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+ 
+ http://www.apache.org/licenses/LICENSE-2.0
+ 
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+ */
+
+import UIKit
+import Reusable
+
+final class EmojiPickerHeaderView: UICollectionReusableView, NibReusable {
+    
+    // MARK: - Properties
+    
+    @IBOutlet private weak var titleLabel: UILabel!
+    
+    // MARK: - Public
+    
+    func update(theme: Theme) {
+        self.backgroundColor = theme.backgroundColor
+        self.titleLabel.textColor = theme.headerTextPrimaryColor
+    }
+    
+    func fill(with title: String) {
+        titleLabel.text = title
+    }
+}