make diagramtab use gtk.scrolledwindow. This is more consistent with the rest of the ui.
make diagramtab use gtk.scrolledwindow. This is more consistent with the rest of the ui.

git-svn-id: 80d932a80928324f4d3d71af042e652e41ead033@2310 a8418922-720d-0410-834f-a69b97ada669

amolenaar/gaphor,amolenaar/gaphor