Start phosh via gnome-session

Start a full gnome-session as gnome-shell does.

To get out of WIP status:

• Check on dev board or qemu

debian/rules

@@ -8,6 +8,12 @@ export DEB_BUILD_MAINT_OPTIONS = hardening=+all
 override_dh_installsystemd:
 	dh_installsystemd --no-start --no-enable
 
+override_dh_systemd_start:
+	dh_systemd_start --no-start --no-restart-after-upgrade
+
+override_dh_systemd_enable:
+	dh_systemd_enable --no-enable
+
 override_dh_auto_test:
 ifeq ($(filter nocheck,$(DEB_BUILD_OPTIONS)),)
 	xvfb-run ninja -C _build test