Ensure every UI has dbus launched and XDG_RUNTIME_DIR set

Those are prerequisite for Anbox (see #327 (closed)) and probably for a lot of GUI applications.

XDG_RUNTIME_DIR: the best is to add elogind to the dependency list. pam_elogind will handle it properly

dbus: export $(dbus-launch) or dbus-run-session $compositor

Progress Report

• postmarketos-ui-gnome (both Wayland and X.org worked already)
• postmarketos-ui-i3wm (!1131 (merged) for XDG_RUNTIME_DIR, don't have dbus)
• postmarketos-ui-mate (had both already)
• postmarketos-ui-phosh (worked already)
• postmarketos-ui-plasma-mobile
• postmarketos-ui-plasma-desktop (don't have dbus)
• postmarketos-ui-sway (a!4815)
• postmarketos-ui-sxmo
• postmarketos-ui-weston (!1118 (merged))
• postmarketos-ui-xfce4 (dbus is started already, !1151 (merged) for XDG_RUNTIME_DIR)

note that postmarketos-ui-shelli and postmarketos-ui-kodi are excluded, as they can't run Anbox or any graphical apps anyway.