gtk: Remove invalid FileChooserDialog constructor

The ..._new_with_backend native function has been removed, so the
corresponding constructor must be removed.
gtk: Remove unused P/Invoke in IconTheme

Now that it uses the Marshaller, IconTheme doesn't call g_strfreev
directly anymore.
gdk: Remove invalid Global.DevicesList method

The native function gdk_devices_list is gone, device handling is now
handled through DeviceManager.
gdk: Remove invalid method and P/Invoke in TextProperty

The native functions gdk_free_text_list and
gdk_text_property_to_utf8_list are gone, and we now need to use StrFreeV
to free the native string list.
gdk: Hide Global.TextPropertyToUtf8ListForDisplay method

It is implemented manually in TextProperty.ToStringListForDisplay.