(Backport) Issue #974: Add visibility icon to GimpItemTreeView header

This patch attempts to backport Jehan's code for issue #974 (closed).

Compared to the dev version, this code adds a call to gtk_tree_view_column_set_alignment () to center the header icons in their cells. It also moves up the code to get the icon_spacing to before it's needed when creating the column headers, and gtk_tree_view_set_headers_visible () is added as well.

What it looks like in 2-10: