overview: Allow to unfullscreen apps

We can bring up the shell chrome by swiping from the top but unfullscreening an app wasn't very obvious. Let's add an icon to make that simpler to discover. We chose the top-left corner as top-right is used for the close button on hover:

Screenshot

As this touches a bit dated phosh code clean up around the edges a bit too.

@snwh maybe you have a suggestion for a better placement or icon?