Make FileListPreprocessor extend AbstractSingleInputPreprocessor

[imagejan]

Currently, FileListPreprocessor implements its own logic to get a suitable File[] input (and it doesn't respect autofill=false annotations):

https://github.com/scijava/scijava-common/blob/46ca0d04c5c86eeb51f9daa9f130b0d7c8decd65/src/main/java/org/scijava/ui/FileListPreprocessor.java#L84-L87

Let's instead extend AbstractSingleInputPreprocessor to harmonize the logic.