XWIKI-6903: Office preview fails for PDF attachments
* Improve the checks done by the office viewer to detect if a media type is supported (i.e. if it can be converted to HTML). The consequence is that PDF attachments won't have the eye icon any more.
* Improve the check for presentation format in office importer, office viewer and WYSIWYG editor server.
* Refactor a bit the office importer because I need access to the document formats registry to be able to detect correctly if a conversion is possible. Note that DocumentFormatRegistry interface is from JODConverter but it is generic. Wrapping DocumentFormatRegistry, DocumentFormat and DocumentFamily with our own interfaces is not worth at this point IMO.
* Remove more mentions of "OpenOffice" from the office modules.