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/window.c b/src/window.c
index 1ba2984..6416e01 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4672,9 +4672,7 @@
     if (curwin->w_width < p_wiw && !curwin->w_p_wfw)
 	win_setwidth((int)p_wiw);
 
-#ifdef FEAT_MOUSE
-    setmouse();			/* in case jumped to/from help buffer */
-#endif
+    setmouse();			// in case jumped to/from help buffer
 
     /* Change directories when the 'acd' option is set. */
     DO_AUTOCHDIR;