Discard changes

This pull request is my humble apology for closing #751 in a fit of premature-i've-fixed-one-accidentally-happiness. Both the quit and file close confirmations should now be centred appropriately above the windows from whence they came - tested both on GNOME and OS X.

For anyone interested in writing their own stuff in Gaffer, the key to getting the window managers to act nicely is to parent your dialogue to the window it is serving - using either GafferUI.Window.addChildWindow(), or in the case of modal dialogues, dialogue.waitFor*( parentWindow= scriptWindow ), which is just a shortcut for doing the former and unparenting again when the dialogue returns.