files-view: Indicate when Ctrl+V cannot be executed

I had to add a new action - "paste_accel" which will handle the accelerator, as the "paste" action is disabled if we are in a read-only folder on line 7611 in files-view.c, thus the Ctrl+V won't be registred. (this is what I found. Maybe there is another way to solve this...)

Also, I'm awaiting for some ideas related to the error message.

Closes #824 (closed)