Adjust for GNOME Shell on mobile

Detect whether Phosh or GNOME Shell on mobile are running by reading XDG_SESSION_DESKTOP, and only show the widgets relevant to the current UI.

On GNOME Shell on mobile it looks like the screenshot below now.

I've considered adjusting the "marker" logic as well (the one that determines when the app should be open, it was recently changed to make it show up once after the Phosh update that introduced swiping for menus). Right now there is only one marker, in the future it would make sense to use separate markers per UI. But it was too much effort right now, given that GNOME Shell on mobile is in prototype state I think we can just do this later on when we really need it. It would also be nice to split this in multiple files, but again, let's do it in the future.