Add callback to EditCustomFieldSelect before rendering values

In some cases, we might want to modify or restrict the values that are
displayed in the custom field select box. This callback allows us to do
that.

share/html/Elements/EditCustomFieldSelect

@@ -212,6 +212,8 @@ $MaxValues => 1
 % $_ = lc $_ foreach @Default;
 % my $selected;
 % my $CFVs = CachedCustomFieldValues($CustomField);
+% $m->callback( CallbackPage => '/Elements/EditCustomFieldSelect', CallbackName => 'BeforeValues',
+%               CustomField => $CustomField, Values => $CFVs, ARGSRef => \%ARGS );
 % my @levels;
 % while ( my $value = $CFVs->Next ) {
 %       my $name = $value->Name;