[displaymanager] Fix naming

- Having a global and a class-local function with the same name
   that do very different things is confusing.

src/modules/displaymanager/displaymanager.conf

@@ -20,9 +20,9 @@ displaymanagers:
 # support configuring a default DE, so the keys are mandatory
 # but their interpretation is up to the DM configuration.
 #
-# Subkeys of *defaultDesktopEnvironment* are:
-#  - *executable* a full path to an executable (manda)
-#  - *desktopFile* a .desktop filename (optional)
+# Subkeys of *defaultDesktopEnvironment* are (all mandatory):
+#  - *executable* a full path to an executable
+#  - *desktopFile* a .desktop filename
 #
 # If this is **not** set, then Calamares will look for installed
 # DE's and pick the first one it finds.

src/modules/displaymanager/main.py

@@ -83,7 +83,7 @@ class DesktopEnvironment:
                 return candidate
         return None
 
-    def find_desktop_environment(self, root_mount_point):
+    def is_installed(self, root_mount_point):
         """
         Check if this environment is installed in the
         target system at @p root_mount_point.
@@ -144,7 +144,7 @@ def find_desktop_environment(root_mount_point):
     """
     libcalamares.utils.debug("Using rootMountPoint {!r}".format(root_mount_point))
     for desktop_environment in desktop_environments:
-        if desktop_environment.find_desktop_environment(root_mount_point):
+        if desktop_environment.is_installed(root_mount_point):
             libcalamares.utils.debug(".. selected DE {!s}".format(desktop_environment.desktop_file))
             return desktop_environment
     return None