Don't clear selection on descendants of tables

We want to remove existing selections during caret navigation, but
clearing the text selection on a full table is non-performant. So
just clear it on the table itself and hope the user agent does the
right thing. If it doesn't, let's ask them to.

src/orca/scripts/web/script_utilities.py

@@ -4103,6 +4103,11 @@ def clearTextSelection(self, obj):
         if self.isDocument(obj):
             return
 
+        # For performance reasons. If clearing the selection on a table doesn't
+        # clear it, file a bug against the user agent in question.
+        if self.getTable(obj):
+            return
+
         try:
             for child in obj:
                 self.clearTextSelection(child)