Only fallback to cmd identifier when needed

Using the cmd identifier is quite problematic.  If it contains null
bytes, it will result in an error farther down the line, causing the
group not to be shown!  This change makes cmd act in the same way as the
window title, in that res_class and res_name are preferred.

It is probably wise to split cmd by null bytes and choose only the first
one, but since I am unsure of all the potential ramifications of such a
change (would it be better to replace null bytes with spaces?) and
because this change fixes the problem for me, I will not include that in
this commit.

dockbarx/dockbar.py

@@ -907,7 +907,7 @@ def __add_window(self, window):
             res_class = window.get_class_group().get_res_class().lower()
             res_name = window.get_class_group().get_name().lower()
         if window.has_name():
-            identifier = res_class or res_name or window.get_name().lower()
+            fallback = window.get_name().lower()
         else:
             #in case window has no name - issue with Spotify
             pid = window.get_pid()
@@ -917,9 +917,10 @@ def __add_window(self, window):
                 raise
             cmd = f.readline()
             if "/" in cmd:
-                identifier = cmd.split("/")[-1]
+                fallback = cmd.split("/")[-1]
             else:
-                identifier = cmd
+                fallback = cmd
+        identifier = res_class or res_name or fallback
         # Special cases
         if identifier in SPECIAL_RES_CLASSES:
             identifier = SPECIAL_RES_CLASSES[identifier]