App menu organisation improvements

In 3.31.x the app menu has been replaced with generic items similar to those found in the dash menu. I think we can consider a few improvements to these:

1. Show Details is in the same menu group as "Quit". I think these are different enough to warrant their own groups.
2. At first I was a bit confused by the first menu item (it was an open window). I think this can be a particular issue if a) there's only one window and b) it has a name that could be read as an action like "Downloads".

For 2, one possibility might be ta add a heading, like "Open Windows". I'm not sure if this is feasible or not, though.