Display ext2/3/4 unicode volume labels properly (#662537)

Note that this enhancement changes GParted to first try the native
tools to determine the volume label for all file systems.  If the
volume label is not found then the fall back method is to use blkid.

There is a small degradation in device refresh times as a result of
this change.

Closes Bug #662537 - Ext4 unicode labels not shown correctly