Restructure TopLeftMenu for accessibility and autofocus it

We use a trick with refs to automatically focus the element, also making use of mx_HiddenFocusable to hide the unnecessary outline. 

The menu itself has been restructured to hide some elements from screen readers (reduce noise) and to have a single unordered list. Screen readers mention when the user "enters" a list, and each item was previously saying "enter list <action>" when it should have just been "<action>".

By focusing automatically, the keyboard can be used to go up/down the menu as may be expected by keyboard users.

src/utils/Accessibility.js

@@ -0,0 +1,19 @@
+/*
+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.
+*/
+
+export function focusCapturedRef(ref) {
+    if (ref) ref.focus();
+}