Please port these changes to 1.8.x and g2 branches.

[Linas Vepstas]

Log Message:
-----------
Please port these changes to 1.8.x and g2 branches.

This patch adds code to allow combo-boxes to run with 
a shared quickfill.  The use of a shared quickfill should
dramatically improve register startup performance for 
systems with a large number of accounts.  See
http://bugzilla.gnome.org/show_bug.cgi?id=120028
for details

Modified Files:
--------------
    gnucash/src/register/register-core:
        combocell.h
    gnucash/src/register/register-gnome:
        combocell-gnome.c