FIX: searchable_fields inheritance from summary_fields includes methods called on fields (fixes #1429)

Using the example:
```php
private static $summary_fields = array(
    'Date.Nice' => 'Date'
);
```

Previously, `DataObject->searchableFields()` would call `$this->relObject('Date.Nice');`, resulting in an error as “Date” is a field, not a relation. After this patch, the field name (“Date”) is extracted and will be included in the array of searchable fields.

Patch also allows users to prevent searchable fields inheriting from summary fields by defining an empty array for their `$searchable_fields`.