patch 8.1.2070: mouse code is spread out

Problem:    Mouse code is spread out.
Solution:   Move mouse terminal code parsing to mouse.c. (Yegappan Lakshmanan,
            closes #4966)
diff --git a/src/proto/mouse.pro b/src/proto/mouse.pro
index fe5cda1..3faf056 100644
--- a/src/proto/mouse.pro
+++ b/src/proto/mouse.pro
@@ -13,6 +13,7 @@
 int jump_to_mouse(int flags, int *inclusive, int which_button);
 void nv_mousescroll(cmdarg_T *cap);
 void nv_mouse(cmdarg_T *cap);
+int check_termcode_mouse(char_u *tp, int *slen, char_u *key_name, char_u *modifiers_start, int idx, int *modifiers);
 int mouse_comp_pos(win_T *win, int *rowp, int *colp, linenr_T *lnump, int *plines_cache);
 win_T *mouse_find_win(int *rowp, int *colp, mouse_find_T popup);
 int vcol2col(win_T *wp, linenr_T lnum, int vcol);