Gtk version again

[Geert Janssens]

Recently there was some discussion on what version of Gtk we should target in 
trunk.

The consensus was 2.18.1, which is what RHEL6 supports. Fedora 14 is shipping 
with 2.22, so targetting 2.24 would exclude those two distributions.

However, I'm having some practical issues here. My goal is to get GnuCash 
ready for Gnome 3 as fast as possible. All migration documentation specifies 
that as a first step, you should target 2.24 and replace all deprecated gtk 
code in the version.

It appears that is not possible if we still have to support 2.18.1 or 2.22.

An example:
We are using GtkComboBoxEntry in several places in the code. This widget is 
deprecated in 2.24, so it should be replaced. The replacement suggestion is a 
regular GtkComboBox, but created with gtk_combo_box_new_with_entry. This will 
give you a GtkComboBox with a GtkEntry child widget, effectively the same as 
the original GtkComboBoxEntry. The problem is, gtk_combo_box_new_with_entry is 
a new function only available starting from 2.24. So in order to replace a 
deprecated widget, I have to use functionality that's not available in the 
older Gtk versions we try to support.

So what choice should I make here:
- give up on Gnome 3 for now (not my preference) ?
- give up on the older Gtk versions (not nice either) ?
- ...

Geert