Split rlc_{dl_,ul_,}window out of rlc.{h,cpp}

This new structure was already used when porting existing osmo-pcu code
to libosmo-gprs-rlcmac, and proved to be much more clear.
It makes no sense to keep both ul and dl window code mixed, since the
code base is mostly different between them, and the developer usually is
only interested in reading/debugging one side at a time.

Change-Id: If21b6f48ef190a718502389cbfde8cbdfc8d7f7a
---
M src/Makefile.am
M src/decoding.cpp
M src/decoding.h
M src/rlc.cpp
M src/rlc.h
A src/rlc_window.cpp
A src/rlc_window.h
A src/rlc_window_dl.cpp
A src/rlc_window_dl.h
A src/rlc_window_ul.cpp
A src/rlc_window_ul.h
M src/tbf.h
M src/tbf_dl.h
M src/tbf_ul.h
14 files changed, 820 insertions(+), 660 deletions(-)