Fix multiple accessibility defects identified by AXE (#10606)

* Mark effects overlay canvas as aria hidden

* Ensure date separators aren't seen as focusable aria separators

* Fix

* Fix font slider not having aria label

* Add missing aria labels

* Fix settings flags setting aria-checked={null}

* Update snapshots

src/components/views/elements/ImageView.tsx

@@ -540,8 +540,9 @@ export default class ImageView extends React.Component<IProps, IState> {
             <FocusLock
                 returnFocus={true}
                 lockProps={{
-                    onKeyDown: this.onKeyDown,
-                    role: "dialog",
+                    "onKeyDown": this.onKeyDown,
+                    "role": "dialog",
+                    "aria-label": _t("Image view"),
                 }}
                 className="mx_ImageView"
                 ref={this.focusLock}