generalized 'firstChars', and some refactoring towards point-free style

generalized firstChars:
`firstChars :: [String] -> [Maybe Char]`

use firstCharsOrElse instead of firstChars which takes a replacement character

some other refactorings for more concise code with point-free function composition
Wenn der Akkumulator hier ein String ist, wäre es dann nicht besser "" zu verwenden als []?