Get rid of the abstract forms

Visual Studio has a serious limitation with Form and Control
inheritance, and not even a custom TypeDescriptorProvider seems to help,
so for now just work around the limitation with non abstract classes.
Updated German language file - thx Siegwarth Binger :)
nl-NL translation: minor corrections/improvements + new tags
nl-NL translation: spcorr + experiment (buttons)
nl-NL translation: preferences 1