feat(dbus): Add GUI configuration handling to interface_config.py

Introduced the `configure_gui` function to manage GUI enabling,
disabling, availability, and status retrieval. Updated the
`AVAILABLE_FEATURES` dictionary to include GUI management functionality,
leveraging systemd and os module operations.

src/revpi_middleware/dbus_middleware1/system_config/interface_config.py

@@ -5,6 +5,7 @@
 from collections import namedtuple
 from enum import Enum
 from logging import getLogger
+from os import X_OK, access
 
 from pydbus import SystemBus
 
@@ -70,6 +71,28 @@ class InterfaceRevpiConfig(DbusInterface):
         return list(AVAILABLE_FEATURES.keys())
 
 
+def configure_gui(action: ConfigActions):
+    gui_available = access("/usr/bin/startx", X_OK)
+
+    if action is ConfigActions.AVAILABLE:
+        return gui_available
+
+    bus = SystemBus()
+    systemd_manager = bus.get(".systemd1")
+
+    if action is ConfigActions.ENABLE:
+        systemd_manager.SetDefaultTarget("graphical.target", True)
+
+    elif action is ConfigActions.DISABLE:
+        systemd_manager.SetDefaultTarget("multi-user.target", True)
+
+    elif action is ConfigActions.STATUS:
+        return systemd_manager.GetDefaultTarget() == "graphical.target"
+
+    else:
+        raise ValueError(f"action {action} not supported")
+
+
 def get_feature(feature: str) -> FeatureFunction:
     if feature not in AVAILABLE_FEATURES:
         raise ValueError(f"feature {feature} does not exist")
@@ -117,7 +140,7 @@ def simple_systemd(action: ConfigActions, unit: str):
 
 
 AVAILABLE_FEATURES = {
-    "gui": False,
+    "gui": FeatureFunction(configure_gui, []),
     "revpi-con-can": False,
     "var-log.mount": False,
     "dphys-swapfile": False,