Extract ReactionDimension out of MessageActionBar

The reactions bits were cluttering up `MessageActionBar` so they have now been
extracted to a separate component.

src/components/views/messages/ReactionDimension.js

@@ -0,0 +1,73 @@
+/*
+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 React from 'react';
+import PropTypes from 'prop-types';
+import classNames from 'classnames';
+
+export default class ReactionDimension extends React.PureComponent {
+    static propTypes = {
+        options: PropTypes.array.isRequired,
+        title: PropTypes.string,
+    };
+
+    constructor(props) {
+        super(props);
+
+        this.state = {
+            selected: null,
+        };
+    }
+
+    onOptionClick = (ev) => {
+        const { key } = ev.target.dataset;
+        this.toggleDimensionValue(key);
+    }
+
+    toggleDimensionValue(value) {
+        const state = this.state.selected;
+        const newState = state !== value ? value : null;
+        this.setState({
+            selected: newState,
+        });
+        // TODO: Send the reaction event
+    }
+
+    render() {
+        const { selected } = this.state;
+        const { options } = this.props;
+
+        const items = options.map(option => {
+            const disabled = selected && selected !== option.key;
+            const classes = classNames({
+                mx_ReactionDimension_disabled: disabled,
+            });
+            return <span key={option.key}
+                data-key={option.key}
+                className={classes}
+                onClick={this.onOptionClick}
+            >
+                {option.content}
+            </span>;
+        });
+
+        return <span className="mx_ReactionDimension"
+            title={this.props.title}
+        >
+            {items}
+        </span>;
+    }
+}