Various ARIA a11y fixes.

Notate RightPanel tabs.
Shorten Screen Reader queues.
Make AccessibleTooltipButton screen reader friendly
Flatten DOM for Sticker button using React Fragments

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/components/structures/FilePanel.js

@@ -126,11 +126,12 @@ const FilePanel = createReactClass({
                     tileShape="file_grid"
                     resizeNotifier={this.props.resizeNotifier}
                     empty={_t('There are no visible files in this room')}
+                    role="tabpanel"
                 />
             );
         } else {
             return (
-                <div className="mx_FilePanel">
+                <div className="mx_FilePanel" role="tabpanel">
                     <Loader />
                 </div>
             );

src/components/structures/NotificationPanel.js

@@ -46,12 +46,13 @@ const NotificationPanel = createReactClass({
                     showUrlPreview={false}
                     tileShape="notif"
                     empty={_t('You have no visible notifications')}
+                    role="tabpanel"
                 />
             );
         } else {
             console.error("No notifTimelineSet available!");
             return (
-                <div className="mx_NotificationPanel">
+                <div className="mx_NotificationPanel" role="tabpanel">
                     <Loader />
                 </div>
             );

src/components/structures/RoomSubList.js

@@ -258,7 +258,7 @@ const RoomSubList = createReactClass({
         const tabindex = this.props.isFiltered ? "0" : "-1";
         return (
             <div className="mx_RoomSubList_labelContainer" title={ title } ref="header">
-                <AccessibleButton onClick={ this.onClick } className="mx_RoomSubList_label" tabIndex={tabindex}>
+                <AccessibleButton onClick={this.onClick} className="mx_RoomSubList_label" tabIndex={tabindex} aria-expanded={!isCollapsed}>
                     { chevron }
                     <span>{this.props.label}</span>
                     { incomingCall }