file-conflict-dialog: port to G_DECLARE* type

Currently we are using the old GObject class declarations, which have two
problems.

One problem is that we cannot use smart pointers like g_autoptr. The other
problem is the boilerplate code generated that makes the code less readable,
so harder to understand.

To fix this use G_DECLARE* type.

https://bugzilla.gnome.org/show_bug.cgi?id=771844