patch 8.0.1479: insert mode completion state is confusing

Problem:    Insert mode completion state is confusing.
Solution:   Move ctrl_x_mode into edit.c.  Add CTRL_X_NORMAL for zero.
diff --git a/src/proto/edit.pro b/src/proto/edit.pro
index f6d645c..1f9e5b7 100644
--- a/src/proto/edit.pro
+++ b/src/proto/edit.pro
@@ -6,6 +6,8 @@
 void change_indent(int type, int amount, int round, int replaced, int call_changed_bytes);
 void truncate_spaces(char_u *line);
 void backspace_until_column(int col);
+int ctrl_x_mode_not_default(void);
+int ctrl_x_mode_not_defined_yet(void);
 int vim_is_ctrl_x_key(int c);
 int ins_compl_add_infercase(char_u *str, int len, int icase, char_u *fname, int dir, int flags);
 void completeopt_was_set(void);