Add SlackFile type.

bindings/java/jni/dataModel_SleuthkitJNI.cpp

bindings/java/jni/dataModel_SleuthkitJNI.h

bindings/java/src/org/sleuthkit/datamodel/AbstractFile.java

...ings/java/src/org/sleuthkit/datamodel/ContentVisitor.java

bindings/java/src/org/sleuthkit/datamodel/FsContent.java

bindings/java/src/org/sleuthkit/datamodel/SleuthkitCase.java

...ava/src/org/sleuthkit/datamodel/SleuthkitItemVisitor.java

bindings/java/src/org/sleuthkit/datamodel/SleuthkitJNI.java

bindings/java/src/org/sleuthkit/datamodel/TskData.java

tsk/fs/tsk_fs.h