gfs2_edit: Split extended display functions into extended.c
    
    The gfs2_edit code currently has three modes of interactive display:
    (1) hex mode, (2) structure display, (3) extended data.  This patch
    breaks the functions related to display mode (3) into its own
    source file: extended.c.  Today, mode (2), structure display, is
    contained in gfs2hex.c, and (1) hex mode is contained in hexedit.c,
    which also contains the bulk of the support code.
    
    rhbz#634623