Properties dialog is not available as soon as more than one file or dir is selected.

Reproducible: Always.

Steps to reproduce:
1. Select more than one file or directory.
2. Right click on a selected item.

Resulting behaviour:
Context menu with `Properties...' entry deactivated.

Expected behaviour:
Context menu with `Properties...' entry fully functional.

Of course, it would be advisable to implement the appropriate dialog too.
It would be very helpful if this dialog could show

* the total size of selected files and directories (and their subdirs),
* the total count of selected files and directories

and it would also be convenient if it still contained methods to change user rights (recursively). The current properties dialog for single selections does an outstanding job, by the way.

Additionally, a few ideas which could improve the yet-to-be dialog even further:

* show separate counters for files and directories (this could also be implemented in the current properties dialog btw)
* show advanced statistics (how many mp3, wav, png, jpg,... files and their size) - this should not be done automatically due to performance...