Merge:  Pick preview image from metadata editor (#171)

Goobi was extended so that a file group can be prodcuced which will only contain a reference to the preview image selected in the metadata editor. To use it, create an additional file group and check the new check box *Link selected preview image only*. Then use the control to set a “representative” (i.e., preview image) for the document in the *pagination* dialog of the metadata editor and export the object. As result, the new file group will be created in the METS/MODS file which only contains the one image that shall be the preview image, i.e.:
``` xml
    <mets:fileGrp USE="PREVIEWIMAGE">                                                                                                                                            
      <mets:file ID="FILE_0003_PREVIEWIMAGE" MIMETYPE="image/jpeg" USE="banner">                                                                                                 
        <mets:FLocat LOCTYPE="URL" xlink:href="http://goobi.example.org/data/1234567/default/00000003.jpg" xmlns:xlink="http://www.w3.org/1999/xlink"/>
      </mets:file>                                                                                                                                                             
    </mets:fileGrp>
```
This special file group will not be linked with the other file groups.

**Required configuration:**

Database update:
``` sql
ALTER TABLE `projectfilegroups` ADD `previewImage` tinyint(1) DEFAULT NULL;
UPDATE `projectfilegroups` SET `previewImage` = '0';
```
In rule set, add to *Metadata types for the physical structure*:
``` xml
	<MetadataType>
		<Name>_representative</Name>
		<language name="de">Vorschaubild</language>
		<language name="en">Preview image</language>
	</MetadataType>
```
Extend *Physical document structure types*:
``` xml
	<DocStrctType>
		<Name>page</Name>
		<!-- ... -->
		<metadata num="1o">_representative</metadata>
	</DocStrctType>
```