Add more checks to backend documents view

[cliffparnitzky]

Add more checks (known from FE) to the backend documents view

The checks could be:

• naming convention
• keywords
• versioning
• global doc name changes
• etc.

Transferred from #3

[cliffparnitzky]

Do not forget to remove the following code after completing this issue and #33: https://github.com/ContaoDMS/dms/blob/master/system/modules/DocumentManagementSystem/dca/tl_dms_documents.php#L571