Add a basic tooltip showing who reacted

This adds a first attempt at tooltip showing who reacted to a message. It
doesn't limit senders or position the tooltip nicely, but the info is there at
least.

Part of https://github.com/vector-im/riot-web/issues/9722

src/components/views/messages/ReactionsRowButtonTooltip.js

@@ -0,0 +1,80 @@
+/*
+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.
+*/
+
+import React from 'react';
+import PropTypes from 'prop-types';
+
+import MatrixClientPeg from '../../../MatrixClientPeg';
+import sdk from '../../../index';
+import { unicodeToShort } from '../../../HtmlUtils';
+import { _t } from '../../../languageHandler';
+
+export default class ReactionsRowButtonTooltip extends React.PureComponent {
+    static propTypes = {
+        // The event we're displaying reactions for
+        mxEvent: PropTypes.object.isRequired,
+        // The reaction content / key / emoji
+        content: PropTypes.string.isRequired,
+        // A Set of Martix reaction events for this key
+        reactionEvents: PropTypes.object.isRequired,
+        visible: PropTypes.bool.isRequired,
+    }
+
+    render() {
+        const Tooltip = sdk.getComponent('elements.Tooltip');
+        const { content, reactionEvents, mxEvent, visible } = this.props;
+
+        const room = MatrixClientPeg.get().getRoom(mxEvent.getRoomId());
+        let tooltipLabel;
+        if (room) {
+            const senders = [];
+            for (const reactionEvent of reactionEvents) {
+                const { name } = room.getMember(reactionEvent.getSender());
+                senders.push(name);
+            }
+            const shortName = unicodeToShort(content) || content;
+            tooltipLabel = <div>{_t(
+                "<reactors/><reactedWith>reacted with %(shortName)s</reactedWith>",
+                {
+                    shortName,
+                },
+                {
+                    reactors: () => {
+                        return <div className="mx_ReactionsRowButtonTooltip_senders">
+                            {senders.join(", ")}
+                        </div>;
+                    },
+                    reactedWith: (sub) => {
+                        return <div className="mx_ReactionsRowButtonTooltip_reactedWith">
+                            {sub}
+                        </div>;
+                    },
+                },
+            )}</div>;
+        }
+
+        let tooltip;
+        if (tooltipLabel) {
+            tooltip = <Tooltip
+                tooltipClassName="mx_ReactionsRowButtonTooltip"
+                visible={visible}
+                label={tooltipLabel}
+            />;
+        }
+
+        return tooltip;
+    }
+}