Allow the copy of file paths to clipboard.

Allow the copy of file paths to clipboard.
This feature is disabled by default, it can be set through
Preferences > Advanced > Behavior > Show "Copy path" ...
When enabled, it adds a menu entry "Copy path" that let the
user copies current selection's paths to X clipboard.
It is very convenient to paste paths to xterm for example.
Patch by Carles Pina i Estany and me.