xdg-desktop-portal should be enabled only environments that actually supports it

Currently D-bus session to xdg-desktop-portal (and extra thread to handle it) is always created for non windowing capable environment (eglfs) just consuming resources and slowing application startup time. There exists undocumented environment variable to disable it QT_NO_XDG_DESKTOP_PORTAL but it should be disabled by default it the system does not support such D-bus service.