Enable drag reordering of tabs for GTK+2.8, and a temporary workaround for GTK+2.6. Add notebook_new_tab and update document_create_new_sci to use it; also remove on_tab_close_clicked

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@479 ea778897-0a13-0410-b9d1-a72fbfd435f5