Move root privilege escalation into gparted wrapper script (#776437)

Move calling of the privilege escalation program which allows a normal
user to run GParted as root from the desktop file into the gparted
wrapper script.  This is in preparation for further changes needed to
grant root access to the X11 display under Wayland.

Don't introduce yet another script so that there aren't two different
names to run GParted by for normal users and root.  Using the same
gparted name but placing two different scripts at /usr/bin/gparted and
/usr/sbin/gparted is not possible because on Arch Linux /usr/sbin is a
symbolic link to /usr/bin.

    Frequently asked questions, Does Arch follow the FHS?
    https://wiki.archlinux.org/index.php/Frequently_asked_questions#Does_Arch_follow_the_FHS.3F

    "Arch Linux follows the file system hierarchy for operating systems
    using the systemd service manager.  See file-hierarchy(7) for an
    explanation of each directory along with their designations.  In
    particular, /bin, /sbin, and /usr/sbin are symbolic links to
    /usr/bin, and /lib (and /lib64 if applicable) are symbolic links to
    /usr/lib".

Bug 776437 - GParted fails to run as root under Wayland