[displaymanager] Also look for Wayland sessions

- Sessions can be X11-sessions (living in xsessions) or Wayland-
   (living in wayland-sessions). Look in both places.
 - Refactor code a little to make it nicer to read.

src/modules/displaymanager/main.py

@@ -51,15 +51,20 @@ class DesktopEnvironment:
         self.executable = exec
         self.desktop_file = desktop
 
+    def find_de_executable(self, root_mount_point):
+        return os.path.exists("{!s}{!s}".format(root_mount_point, self.executable))
+    
+    def find_de_session(self, root_mount_point):
+        x11_sessions = "{!s}/usr/share/xsessions/{!s}.desktop".format(root_mount_point, self.desktop_file)
+        wayland_sessions = "{!s}/usr/share/wayland-sessions/{!s}.desktop".format(root_mount_point, self.desktop_file)
+        return os.path.exists(x11_sessions) or os.path.exists(wayland_sessions)
+    
     def find_desktop_environment(self, root_mount_point):
         """
         Check if this environment is installed in the
         target system at @p root_mount_point.
         """
-        return (
-            os.path.exists("{!s}{!s}".format(root_mount_point, self.executable)) and
-            os.path.exists("{!s}/usr/share/xsessions/{!s}.desktop".format(root_mount_point, self.desktop_file))
-            )
+        return find_de_executable(root_mount_point) and find_de_session(root_mount_point)
 
 
 desktop_environments = [