Friendly attribution hint in the statusbar when copying between two documents

When copying/importing from one document to another there is no hint with regard to the attribution and license information. My suggestion is to add a simple message to the statusbar with a message text that depends on respective license.

### Copying from a document with CC-BY license to a document with CC-BY-SA
"Copying from "doc1.svg" to "doc2.svg" complies with the license information."

### Other way around CC-BY-SA to CC-BY
"Copying from "doc1.svg" to "doc2.svg" _does not_ comply with the license information set in the respective documents."

### Copying from or to a document that has not set any attribution or license information
""docX.svg" has no license information set to it. Check the license information under document properties->Metadata/License"

### License set but not CC or PD
"Licenses are set but the relation between them is unknown. Check the license information under document properties->Metadata/License"

If time permits
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1. Add more detailed information in the "Document Properties"->Metadata/License tabs after someone has copied from one document to another. (If suitable add tag to inkscape svg namespace to be able to keep this information between save and load, I'm doubtful about adding tag for this.)

This could be links to respective license and a longer explanation what differs between two CC licenses.

2. In "Document Properties"->Metadata/License add user interface to copy and merge Metadata and License information between two documents.