Collect gdate related functions into a common file.

[David Hampton]

Log Message:
-----------
Collect gdate related functions into a common file.  Add a bunch of
gdate modification functions.

Tags:
----
gnucash-gnome2-dev

Modified Files:
--------------
    gnucash:
        ChangeLog
    gnucash/src/app-utils:
        gnc-helpers.c
        gnc-helpers.h
    gnucash/src/core-utils:
        Makefile.am
    gnucash/src/engine:
        gnc-date.c
        gnc-date.h
    gnucash/src/gnome:
        druid-acct-period.c
        druid-loan.c

Added Files:
-----------
    gnucash/src/core-utils:
        gnc-gdate-utils.c
        gnc-gdate-utils.h