appMenu: Add heading to windows section

It may not be immediately obvious that the windows section is a
list of open application windows, as titles like "Downloads" can
easily be confused with an action. Add a section heading to avoid

1 job for <span class="ref-name">wip/fmuellner/app-menu-refinement</span> in 2 minutes and 16 seconds
Status Job ID Name Coverage
  Source Check
passed #212729