patch 8.1.2062: the mouse code is spread out

Problem:    The mouse code is spread out.
Solution:   Move all the mouse code to mouse.c. (Yegappan Lakshmanan,
            closes #4959)
diff --git a/src/Make_morph.mak b/src/Make_morph.mak
index a69eda0..6b549df 100644
--- a/src/Make_morph.mak
+++ b/src/Make_morph.mak
@@ -74,6 +74,7 @@
 	message.c						\
 	misc1.c							\
 	misc2.c							\
+	mouse.c							\
 	move.c							\
 	normal.c						\
 	ops.c							\