Try to avoid crashes in builds against older gtk3 versions
Debian stable, currently wheezy, has gtk-3.4 (fedora 20 has 3.10). This crashes in cairo code when the cursor leaves e.g. an editor or device tool during drag'n'drop. So amend 9f96681 to reduce the chances of this happening.
NB wheezy always crashes if you try to alt-tab out of 4Pane during drag'n'drop. For wheezy, gtk3 should be considered "at your own risk!".