displaymanager: Fix the default_desktop_environment option.

Do not use a YAML list when only a single entry actually works. (It was
just silently using the last one.)

Create the default_desktop_environment DesktopEnvironment tuple. Before,
the setting was silently ignored altogether.

src/modules/displaymanager/displaymanager.conf

@@ -12,7 +12,7 @@ displaymanagers:
   - kdm
 
 #Enable followed settings to force a destop environment in your displaymanager configuration file
-#default_desktop_environment:  
-#  - executable: "startkde"
+#default_desktop_environment:
+#    executable: "startkde"
 #    desktop_file: "plasma"
 

src/modules/displaymanager/main.py

@@ -203,9 +203,8 @@ def run():
     root_mount_point = libcalamares.globalstorage.value("rootMountPoint")
     
     if "default_desktop_environment" in libcalamares.job.configuration:
-        for entry in libcalamares.job.configuration["default_desktop_environment"]:
-               executable = entry["executable"]
-               desktop_file = entry["desktop_file"]
+        entry = libcalamares.job.configuration["default_desktop_environment"]:
+        default_desktop_environment = DesktopEnvironment(entry["executable"], entry["desktop_file"])
 
     if default_desktop_environment is None:
         default_desktop_environment = find_desktop_environment(root_mount_point)