Poll function for File Rename that ensures that exactly one file
browser item is selected.
This is just a custom poll function for FILE_OT_rename.
It currently only checks if File Browser is in use. Therefore it is enabled when nothing is selected and just fails silently if you select it. Similarly it is enabled when more than one file is selected and therefore hard to tell which one it will operate on.
The patch checks if File Browser is active, not in Asset browser, that we are not browsing in a library, and that exactly one file is selected.

