Add quick reaction buttons in tooltip

This adds the set of quick reactions as buttons in a new tooltip accessed via
the react action in the message action bar.

Part of https://github.com/vector-im/riot-web/issues/9753

res/css/views/messages/_ReactionTooltipButton.scss

@@ -0,0 +1,24 @@
+/*
+Copyright 2019 The Matrix.org Foundation C.I.C.
+
+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.
+*/
+
+.mx_ReactionTooltipButton {
+    font-size: 16px;
+    user-select: none;
+}
+
+.mx_ReactionTooltipButton_selected {
+    opacity: 0.4;
+}