I18n: Add suport for labels from modifiers' subpanels.

Was a bit oif a struggle since those functions take a first string which
is not our label, but should work fine now.

Reported/detected as part of D15418.

release/scripts/modules/bl_i18n_utils/settings.py

@@ -251,6 +251,11 @@ PYGETTEXT_KEYWORDS = (() +
     tuple(("{}\\((?:[^\"',]+,){{2}}\\s*" + _msg_re + r"\s*(?:\)|,)").format(it)
           for it in ("BKE_modifier_set_error",)) +
 
+    # This one is a tad more risky, but in practice would not expect a name/uid string parameter
+    # (the second one in those functions) to ever have a comma in it, so think this is fine.
+    tuple(("{}\\((?:[^,]+,){{2}}\\s*" + _msg_re + r"\s*(?:\)|,)").format(it)
+          for it in ("modifier_subpanel_register", "gpencil_modifier_subpanel_register")) +
+
     # bUnitDef unit names.
     # NOTE: regex is a bit more complex than it would need too. Since the actual
     # identifier (`B_UNIT_DEF_`) is at the end, if it's simpler/too general it