Make the preferences dialog a modal to the main window

https://bugzilla.gnome.org/show_bug.cgi?id=722874

Signed-off-by: Meg Ford <meg387@gmail.com>
Set the title of the preferences to "Preferences"

https://bugzilla.gnome.org/show_bug.cgi?id=722876

Signed-off-by: Meg Ford <meg387@gmail.com>