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):

scijava-common/src/main/java/org/scijava/ui/FileListPreprocessor.java

Lines 84 to 87 in 46ca0d0

	if (result != null) {
	// second File parameter; abort
	return null;
	}

Let's instead extend AbstractSingleInputPreprocessor to harmonize the logic.