Add pkgfile support for listing noninstalled packages content

Hello,

This patch adds support for listing content for noninstalled packages using pkgfile. The user will select a noninstalled package from the list, and, if she has pkgfile installed on her system, Octopi will show the package content by executing `pkgfile -l <package>`, otherwise, Octopi will show the message **This package is not installed, to view the content of this package, you need to install "pkgfile"**.

For installed packages, Octopi will still use pacman for retrieve the package content.

This patch is working but I think there are some unimplemented things to take into account:
 1. When pkgfile is not installed, the error message should be showed on a more elegant way rather than creating an item for the files treeview to showing it.
 2. Octopi should warn user when pkgfile database is out-to-date (for example, 30 hours old) and maybe a button to update pkgfile database on the spot and/or an explanation to configure a scheduled job to update it automatically.

Cheers!