Implement icon text column using native GTK renderers in wxDVC.

Implement icon text column using native GTK renderers in wxDVC.

This has a possible advantage of a more native look and feel (although it's
hard to tell the difference between drawing the icon ourselves and how the
default GTK+ renderer does it to be honest) and a very real advantage of
allowing to edit in place cells with icons. It also reduces code duplication
in GTK implementation.

Modify the sample to make the icon-text column in the list model editable to
show that it works. This required storing the values of the second column as
well, so do it in its own array and to avoid calling it "m_array2", rename the
existing m_array to m_textColValues (which accounts for most of the diff in
the sample) and call the new one m_iconColValues.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62423 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775