org-capture.el: Give a default value when prompting for a property

* lisp/org-capture.el (org-capture-templates)
(org-capture-fill-template): Enable declaring a default value
when prompting for a property during capture.

* lisp/org.el (org-read-property-value): Add parameter DEFAULT
for an initial/default/suggested property value.

* doc/org-manual.org: Document declaring a default value for a
prompted property during capture.

doc/org-manual.org

@@ -7858,7 +7858,8 @@ here:
 
 - =%^{PROP}p= ::
 
-  Prompt the user for a value for property {{{var(PROP)}}}.
+  Prompt the user for a value for property {{{var(PROP)}}}.  You may
+  specify a default value with =%^{PROP|default}=.
 
 - =%^{PROMPT}= ::
 

lisp/org-capture.el

@@ -332,6 +332,8 @@ be replaced with content and expanded:
   %^C         Interactive selection of which kill or clip to use.
   %^L         Like %^C, but insert as link.
   %^{prop}p   Prompt the user for a value for property `prop'.
+              A default value can be specified like this:
+              %^{prop|default}p.
   %^{prompt}  Prompt the user for a string and replace this sequence with it.
               A default value and a completion table can be specified like this:
               %^{prompt|default|completion2|completion3|...}.
@@ -1787,7 +1789,8 @@ The template may still contain \"%?\" for cursor positioning."
 					   (setq l (org-up-heading-safe)))
 					 (if l (point-marker)
 					   (point-min-marker)))))))
-			    (value (org-read-property-value prompt pom)))
+			    (value
+			     (org-read-property-value prompt pom default)))
 		       (org-set-property prompt value)))
 		    ((or "t" "T" "u" "U")
 		     ;; These are the date/time related ones.

lisp/org.el

@@ -13300,11 +13300,12 @@ This is computed according to `org-property-set-functions-alist'."
   (or (cdr (assoc property org-property-set-functions-alist))
       'org-completing-read))
 
-(defun org-read-property-value (property &optional pom)
+(defun org-read-property-value (property &optional pom default)
   "Read value for PROPERTY, as a string.
 When optional argument POM is non-nil, completion uses additional
 information, i.e., allowed or existing values at point or marker
-POM."
+POM.
+Optional argument DEFAULT provides a default value for PROPERTY."
   (let* ((completion-ignore-case t)
 	 (allowed
 	  (or (org-property-get-allowed-values nil property 'table)
@@ -13320,7 +13321,8 @@ POM."
      (if allowed
 	 (funcall set-function
 		  prompt allowed nil
-		  (not (get-text-property 0 'org-unrestricted (caar allowed))))
+		  (not (get-text-property 0 'org-unrestricted (caar allowed)))
+		  default nil default)
        (let ((all (mapcar #'list
 			  (append (org-property-values property)
 				  (and pom