Add tooltip for format buttons

res/css/views/rooms/_MessageComposerFormatBar.scss

@@ -79,3 +79,9 @@ limitations under the License.
         mask-image: url('$(res)/img/format/code.svg');
     }
 }
+
+.mx_MessageComposerFormatBar_buttonTooltip {
+    white-space: nowrap;
+    font-size: 12px;
+    font-weight: 600;
+}

src/components/views/rooms/MessageComposerFormatBar.js

@@ -17,6 +17,8 @@ limitations under the License.
 import React from 'react';
 import PropTypes from 'prop-types';
 import { _t } from '../../../languageHandler';
+import sdk from '../../../index';
+
 
 export default class MessageComposerFormatBar extends React.PureComponent {
     static propTypes = {
@@ -67,13 +69,20 @@ class FormatButton extends React.PureComponent {
     }
 
     render() {
+        const InteractiveTooltip = sdk.getComponent('elements.InteractiveTooltip');
         const className = `mx_MessageComposerFormatBar_button mx_MessageComposerFormatBar_buttonIcon${this.props.icon}`;
+        const tooltipContent = (
+            <div className="mx_MessageComposerFormatBar_buttonTooltip">{this.props.label}</div>
+        );
+
         return (
+            <InteractiveTooltip content={tooltipContent}>
                 <span aria-label={this.props.label}
                    role="button"
                    onClick={this.props.onClick}
                    className={className}>
                 </span>
+            </InteractiveTooltip>
         );
     }
 }