Change Markdown buttons to onClick

`AccessibleButton` expects click handlers, which we can use here, as long we
make some additional changes to avoid race processing the new state (see
subsequent patch).

Fixes element-hq/element-web#8866

src/components/views/rooms/MessageComposer.js

@@ -501,7 +501,7 @@ export default class MessageComposer extends React.Component {
                         { formatButtons }
                         <div style={{ flex: 1 }}></div>
                         <AccessibleButton className="mx_MessageComposer_formatbar_markdown mx_MessageComposer_markdownDisabled"
-                            onMouseDown={this.onToggleMarkdownClicked}
+                            onClick={this.onToggleMarkdownClicked}
                             title={_t("Markdown is disabled")}
                         />
                         <AccessibleButton element="img" title={_t("Hide Text Formatting Toolbar")}

src/components/views/rooms/MessageComposerInput.js

@@ -1603,7 +1603,7 @@ export default class MessageComposerInput extends React.Component {
                 </div>
                 <div className={className}>
                     <AccessibleButton className={markdownClasses}
-                        onMouseDown={this.onMarkdownToggleClicked}
+                        onClick={this.onMarkdownToggleClicked}
                         title={this.state.isRichTextEnabled ? _t("Markdown is disabled") : _t("Markdown is enabled")}
                     />
                     <Editor ref={this._collectEditor}