Update German translation (Preview-mode)

Translate new Preview-mode documentation.
Look through 'fuzzy' entries.
Add note about default session to Help
Merge pull request #229 from uliska/de-translate-preview

Update German translation (Preview-mode)
Merge pull request #230 from uliska/session-doc

Add note about default session to Help
space in l10n string
pot/po/nl
correct subtle bug
smarter
growing text widget