Add icon for foldbar visibility control

Fixes #2525

core/ui/ViewToolbar/fold-bar.tid

@@ -1,6 +1,6 @@
 title: $:/core/ui/Buttons/fold-bar
 tags: $:/tags/ViewToolbar
-caption: {{$:/language/Buttons/Fold/FoldBar/Caption}}
+caption: {{$:/core/images/chevron-up}} {{$:/language/Buttons/Fold/FoldBar/Caption}}
 description: {{$:/language/Buttons/Fold/FoldBar/Hint}}
 
 <!-- This dummy toolbar button is here to allow visibility of the fold-bar to be controlled as if it were a toolbar button -->