add button to close all open applications to phosh

I think people will come to the situation where they don't bother what apps are currently open and just want to close them all and see the app drawer full-screen.

Where to add a button to close all is up to the design team. My idea would be to the right of the last/rightmost app, visible as soon as there are 2+ open apps.

I opened in os-issues because I don't know what additions would be needed for phosh to know what to close.