Enable the Forward button after the user closes the dialog

Currently, after the user closes the dialog, the Forward button ("button_next") is disabled which prevents the user from completing the install. This extra line re-enables (sensitivises? :-P ) the Forward button after the user Ok's the dialog.

I have no idea if this is the best way to do this - I haven't looked at GTK code before.
Merge pull request #15 from MrHippo/patch-1

Enable the Forward button after the user closes the dialog during a custom install