patch 8.1.1766: code for writing session file is spread out

Problem:    Code for writing session file is spread out.
Solution:   Put it in one file. (Yegappan Lakshmanan, closes #4728)
diff --git a/Filelist b/Filelist
index c04eda6..d4d390c 100644
--- a/Filelist
+++ b/Filelist
@@ -88,6 +88,7 @@
 		src/regexp.h \
 		src/screen.c \
 		src/search.c \
+		src/session.c \
 		src/sha256.c \
 		src/sign.c \
 		src/sound.c \
@@ -218,6 +219,7 @@
 		src/proto/regexp.pro \
 		src/proto/screen.pro \
 		src/proto/search.pro \
+		src/proto/session.pro \
 		src/proto/sha256.pro \
 		src/proto/sign.pro \
 		src/proto/sound.pro \