Send sticker: Create StickerPickerVC and use it from the RoomVC

https://github.com/vector-im/riot-ios/issues/1860

Riot/ViewController/Widgets/StickerPickerViewController.h

@@ -0,0 +1,21 @@
+/*
+ Copyright 2018 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 "WidgetViewController.h"
+
+@interface StickerPickerViewController : WidgetViewController
+
+@end