Default TreeDropdown to "Title" search if $labelField isn't in DB

This is a workaround in order to ensure the field stays operational
for SiteTree and File records with the new $showSearch=true default.
Previously it was necessary to use setSearchCallback(), otherwise
the SQL query would fail. One limitation to keep this change generic
is that "MenuTitle" won't be used to search, since its SiteTree specific,
while the "Title" and "Name" fields are generally regarded as
model conventions (e.g. they're used in DataObject->getTitle() as well).

See https://github.com/silverstripe/silverstripe-framework/pull/2364