Add setting to enable widget screenshots (if widgets declare support)

Fixes the remainder of https://github.com/vector-im/riot-web/issues/6708

Signed-off-by: Travis Ralston <travpc@gmail.com>

src/components/structures/UserSettings.js

@@ -80,6 +80,7 @@ const SIMPLE_SETTINGS = [
     { id: "TextualBody.disableBigEmoji" },
     { id: "VideoView.flipVideoHorizontally" },
     { id: "TagPanel.disableTagPanel" },
+    { id: "enableWidgetScreenshots" },
 ];
 
 // These settings must be defined in SettingsStore