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/EffectsOverlay.tsx

@@ -90,6 +90,7 @@ const EffectsOverlay: FunctionComponent<IProps> = ({ roomWidth }) => {
                 top: 0,
                 right: 0,
             }}
+            aria-hidden={true}
         />
     );
 };

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}

src/components/views/elements/SettingsFlag.tsx

@@ -62,7 +62,7 @@ export default class SettingsFlag extends React.Component<IProps, IState> {
     }
 
     private getSettingValue(): boolean {
-        return SettingsStore.getValueAt(
+        return !!SettingsStore.getValueAt(
             this.props.level,
             this.props.name,
             this.props.roomId ?? null,

src/components/views/elements/Slider.tsx

@@ -35,6 +35,8 @@ interface IProps {
 
     // Whether the slider is disabled
     disabled: boolean;
+
+    label: string;
 }
 
 const THUMB_SIZE = 2.4; // em
@@ -77,6 +79,7 @@ export default class Slider extends React.Component<IProps> {
                     disabled={this.props.disabled}
                     step={this.props.step}
                     autoComplete="off"
+                    aria-label={this.props.label}
                 />
                 {selection}
             </div>