i18n(de-DE): improve some translations

[gabschne]

I tried to improve some translations and fixed a typo:

• Normally in German, word combinations are simply concatinated, but the use of hyphens is possible for better readability.
• A direct translation of headers would be "Köpfe", however that sounds a little bit strange to me in the context of a developer tool and "Header(s)" seems to be Germanized so I used that. Either way, "Überschriften*" is definitely a wrong translation here.

This PR is related to #348.

[ghost]

DeepCode's analysis on #d5cb66 found:

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