vim-patch:8.1.0614: placing signs can be complicated

Problem:    Placing signs can be complicated.
Solution:   Add functions for defining and placing signs.  Introduce a group
            name to avoid different plugins using the same signs. (Yegappan
            Lakshmanan, closes vim/vim#3652)
162b71479b

src/nvim/sign_defs.h

@@ -12,10 +12,15 @@ struct signlist
     int id;             // unique identifier for each placed sign
     linenr_T lnum;      // line number which has this sign
     int typenr;         // typenr of sign
+    char_u *group;		   // sign group
+    int priority;	     // priority for highlighting
     signlist_T *next;   // next signlist entry
     signlist_T *prev;   // previous entry -- for easy reordering
 };
 
+// Default sign priority for highlighting
+#define SIGN_DEF_PRIO	10
+
 // type argument for buf_getsigntype() and sign_get_attr()
 typedef enum {
   SIGN_ANY,