Moved parts of UnCore.h to UnCoreSerialize.cpp
Implemented buffering for FFileArchive.
Export:
- New system for tracking duplicated Export() calls for the same object (when some object was already exported by link from a different package).
  Previous code had bug with batch export from GUI: objects was tracked by pointers, but the whole UObject set was released after each package
  has been exported., so object pointers became invalid.
- Resetting exported object list after each export operation, so the same object could be exported multiple times from UI when user wants.
- Setting export directory globally: previously Ctrl+X didn't use "-out" option.
- When user starts umodel and exports some packages from "select package" UI, then press "Cancel", umodel window was popped up anyway.

Other:
- FFileArchive::f wasn't initialized (caused assertion in Open() -> IsOpen() when trying to save a screenshot).
- build.sh: don't increment revision number anymore - there's no point to do that.