Only override AWT-related methods in the GenericDialog

Paul van Schayck reported that a certain macro did not work for him in headless mode. The reason is that a non-AWT method was introduced into the `GenericDialog`, making it even harder to maintain the `HeadlessGenericDialog` hack than it already is, because the assumption of clean separation between graphical user interface and other logic was broken (`parseDouble()` is hardly specific to graphical dialogs, it desperately wanted to live in `ij.util.Tools` but was denied).

This PR addresses that problem.