Currently, the Paste action is disabled in Read-Only
directories (both the Ctrl+V accelerator and the "Paste"
option from Context Menu), but the user isn't informed
that the operation cannot be executed if he uses Ctrl+V.

A pop-over should inform the user that the Paste action
couldn't be completed if Ctrl+V accelerator was used.

This patch solves this by adding a new action and mapping
the Ctrl+V accelerator to that.

Closes GNOME/nautilus#824