A11y: repurpose more divs into AccessibleButtons.

With this more of the
controls that look like buttons can be operated via the keyboard and
navigated to by screen reader users. This includes editor buttons such
as File upload, Audio / Video call, Right pannel hide button, Jump to
the bottom timeline button, and some more buttons found in the user
settings.
Also I have added alt texts to some images that in turn label buttons
which these happen to be packed in and removed some untranslated alt
texts from decorative non-actionable images that might add more
verbosity when talking about screen reader user experience.

src/components/structures/RoomStatusBar.js

@@ -223,14 +223,15 @@ module.exports = React.createClass({
             );
         }
 
+            const AccessibleButton = sdk.getComponent("elements.AccessibleButton");
         if (!this.props.atEndOfLiveTimeline) {
             return (
-                <div className="mx_RoomStatusBar_scrollDownIndicator"
+                <AccessibleButton className="mx_RoomStatusBar_scrollDownIndicator"
                         onClick={this.props.onScrollToBottomClick}>
                     <img src="img/scrolldown.svg" width="24" height="24"
                         alt={_t("Scroll to bottom of page")}
                         title={_t("Scroll to bottom of page")} />
-                </div>
+                </AccessibleButton>
             );
         }
 
@@ -385,7 +386,7 @@ module.exports = React.createClass({
         }
 
         return <div className="mx_RoomStatusBar_connectionLostBar">
-            <img src="img/warning.svg" width="24" height="23" title={_t("Warning")} alt={_t("Warning")} />
+            <img src="img/warning.svg" width="24" height="23" title={_t("Warning")} alt="" />
             <div>
                 <div className="mx_RoomStatusBar_connectionLostBar_title">
                     { title }
@@ -485,7 +486,9 @@ module.exports = React.createClass({
                 <div className="mx_RoomStatusBar_indicator">
                     { indicator }
                 </div>
-                { content }
+                <div role="alert">
+                    { content }
+                </div>
             </div>
         );
     },