Register types to fix casting

[jwharm]

This will fix #18

[JFronny]

This causes a regression with cross-platform compatibility, since a non-existing type will now cause a NPE while type registration is ongoing (EG Cannot invoke "java.lang.invoke.MethodHandle.invokeExact()" because "org.gtk.gio.DesktopAppInfo$DowncallHandles.g_desktop_app_info_get_type" is null on Windows)

[jwharm]

I will add a try/catch around each getType call to suppress the NPE. (somewhere later this week probably)

If you generate GIR files on Windows from a GTK build folder, those files should only contain valid types for the platform. Or use the repository mentioned by badcel in #16.

[JFronny]

I'm currently hacking together a workaround by inserting a null check, which seems to work.