patch 8.1.2362: cannot place signs in a popup window

Problem:    Cannot place signs in a popup window. (Maxim Kim)
Solution:   Use the group prefix "PopUp" to specify which signs should show up
            in a popup window. (closes #5277)
diff --git a/src/sign.c b/src/sign.c
index 134edf0..f3a1f9d 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -138,7 +138,20 @@
     return ((group != NULL && STRCMP(group, "*") == 0)
 	    || (group == NULL && sign->se_group == NULL)
 	    || (group != NULL && sign->se_group != NULL
-				 && STRCMP(group, sign->se_group->sg_name) == 0));
+			      && STRCMP(group, sign->se_group->sg_name) == 0));
+}
+
+/*
+ * Return TRUE if "sign" is to be displayed in window "wp".
+ * If the group name starts with "PopUp" it only shows in a popup window.
+ */
+    static int
+sign_group_for_window(sign_entry_T *sign, win_T *wp)
+{
+    int for_popup = sign->se_group != NULL
+			&& STRNCMP("PopUp", sign->se_group->sg_name, 5) == 0;
+
+    return WIN_IS_POPUP(wp) ? for_popup : !for_popup;
 }
 
 /*
@@ -484,8 +497,7 @@
 
 	if (sign->se_lnum == lnum
 # ifdef FEAT_TEXT_PROP
-		&& sign_in_group(sign, (char_u *)"popupmenu")
-					  == (WIN_IS_POPUP(wp) ? TRUE : FALSE)
+		&& sign_group_for_window(sign, wp)
 # endif
 		)
 	{
@@ -2645,8 +2657,7 @@
     sign_entry_T *sign = wp->w_buffer->b_signlist;
 
 # ifdef FEAT_TEXT_PROP
-    while (sign != NULL && sign_in_group(sign, (char_u *)"popupmenu")
-					  == (WIN_IS_POPUP(wp) ? FALSE : TRUE))
+    while (sign != NULL && !sign_group_for_window(sign, wp))
 	sign = sign->se_next;
 # endif
     return sign;