[cuegui] Allow non-alphanumeric characters in limits (eg. _ and -) wh…

…en editing layers (AcademySoftwareFoundation#1616)

**Link the Issue(s) this Pull Request is related to.**
Fixes AcademySoftwareFoundation#1565 

**Summarize your change.**
Removes the check for alphanumeric characters in limits when editing a
layer in cuegui

Co-authored-by: Diego Tavares <dtavares@imageworks.com>

cuegui/cuegui/LimitSelectionWidget.py

@@ -63,4 +63,4 @@ def get_selected_limits(self):
         @rtype: list<str>
         """
         limit_names = [str(limit.text()) for limit in self.limits.checkedBoxes()]
-        return [limit.strip() for limit in limit_names if limit.strip().isalnum()]
+        return [limit.strip() for limit in limit_names if limit.strip()]