patch 8.1.1928: popup windows don't move with the text when making changes

Problem:    Popup windows don't move with the text when making changes.
Solution:   Add the 'textprop" property to the popup window options, position
            the popup relative to a text property. (closes #4560)
            No tests yet.
diff --git a/src/proto/textprop.pro b/src/proto/textprop.pro
index f33c7a4..028e3d4 100644
--- a/src/proto/textprop.pro
+++ b/src/proto/textprop.pro
@@ -1,7 +1,9 @@
 /* textprop.c */
+int find_prop_type_id(char_u *name, buf_T *buf);
 void f_prop_add(typval_T *argvars, typval_T *rettv);
 void prop_add_common(linenr_T start_lnum, colnr_T start_col, dict_T *dict, buf_T *default_buf, typval_T *dict_arg);
 int get_text_props(buf_T *buf, linenr_T lnum, char_u **props, int will_change);
+int find_visible_prop(win_T *wp, int type_id, int id, textprop_T *prop, linenr_T *found_lnum);
 proptype_T *text_prop_type_by_id(buf_T *buf, int id);
 void f_prop_clear(typval_T *argvars, typval_T *rettv);
 void f_prop_list(typval_T *argvars, typval_T *rettv);