tbf/rlc: Move the parsing of RBB to Decoding, move window marking out

Move the parsing of the bitbmap out of the TBF code into Decoding.
Move the updating of the V_B into the V_B class. Add some comments
about handling the mod_sns, mod_sns_half parameters by using template
code.
diff --git a/src/decoding.h b/src/decoding.h
index 0590eb4..31be032 100644
--- a/src/decoding.h
+++ b/src/decoding.h
@@ -28,4 +28,6 @@
 	static int tlli_from_ul_data(const uint8_t *data, uint8_t len,
 					uint32_t *tlli);
 	static uint8_t get_ms_class_by_capability(MS_Radio_Access_capability_t *cap);
+
+	static void extract_rbb(const uint8_t *rbb, char *extracted_rbb);
 };