Makes enums/structs/tables unsubclassable (final or sealed) and
prevents instantiation of enum classes (which are solely static
constants).

Tested (Mac OS 10.10.2):
1. run flattests
2. cd tests && ../flatc -c monster_test.fbs && ../flatc -j
monster_test.fbs && ../flatc -g monster_test.fbs && ../flatc -n
monster_test.fbs  # Note deltas for C# and Java.
3. ./JavaTest.sh

**Breaking api change**

Change-Id: Ie008c941c36d212690da58ddc72c9b228eb7a093