Add fallback method for specifying GParted icon (#695279)

On RHEL / CentOS 5.9 GParted couldn't set an icon as the
set_default_icon_name() method is not available.  See commit [1] for
details.

Re-add the old set_icon_from_file() method as a fallback and re-install
a GParted pixmap as was used before commit [2].

Commit [1]:
    a0421078
    Only use Gtk::Window::set_default_icon_name method when available

Commit [2]:
    f5a80bc9
    Enabled GParted to use themed app icon (Tango theme)

Bug #695279 - GParted doesn't compile on RHEL / CentOS 5.9