language selection added to 'Preferences' dialog. Refs: #89.
As proposed by Helmut Zinn in CPC 1.7 rc6 with minor adaptations:

1. Any reference to CPC packages removed. Note that any module that needs to be
informed about language selection events can use Dialog.RegisterLangNotifier.
2. Language selection without a special notifier that switches the language immediately.
Instead, the language selection is applied when pressing the OK button.

Signed-off-by: Josef Templ <josef.templ@gmail.com>