main/postmarketos-ui-{phosh,plasma-mobile}: use xdg-user-dirs

xdg-user-dirs will automatically create the XDG user directories, things like ~/Music and ~/Documents, and even translate them and such when required. I added them to _pmb_recommends as some people do note like the things XDG does so it can be removed without harm.

Tested it in Qemu and the directories are indeed created. I would add this to postmarketos-ui-{plasma-desktop,gnome} as well but those depend on the gnome and plasma meta packages which seem like better places to put these things so Alpine benefits from them as well, so I'll MR it to Alpine.

Note that when cherry-picking this to stable, the xdg-user-dirs package from Alpine will have to be introduced as a new package in the v20.05 branch as it doesn't exist in Alpine 3.12.

Fixes #680 (closed)