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/proto.h b/src/proto.h
index efedc0e..0081ad0 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -175,6 +175,7 @@
 /* Use our own qsort(), don't define the prototype when not used. */
 void qsort(void *base, size_t elm_count, size_t elm_size, int (*cmp)(const void *, const void *));
 #endif
+# include "mouse.pro"
 # include "move.pro"
 # include "mbyte.pro"
 # include "normal.pro"