Allow setting a default category.

Needs some testing, I am not sure how to solve the case now where "no category" shall be used. It will always fall back to the default category.
Probably if someone uses a default category, that's the intended behaviour?

include/functions_entries.inc.php

@@ -627,7 +627,7 @@ function &serendipity_fetchEntryProperties($id) {
  * @param   string  The ACL artifact condition. If set to "write" only categories will be shown that the author can write to. If set to "read", only categories will be show that the author can read or write to.
  * @return  array   Returns the array of categories
  */
-function &serendipity_fetchCategories($authorid = null, $name = null, $order = null, $artifact_mode = 'write') {
+function &serendipity_fetchCategories($authorid = null, $name = null, $order = null, $artifact_mode = 'write', $flat = false) {
     global $serendipity;
 
     if ($name === null) {
@@ -716,6 +716,16 @@ function &serendipity_fetchCategories($authorid = null, $name = null, $order = n
     $ret =& serendipity_db_query($querystring);
     if (is_string($ret)) {
         echo "Query failed: $ret";
+    } else {
+        if ($flat) {
+          $cats = serendipity_walkRecursive($ret, 'categoryid', 'parentid', VIEWMODE_THREADED);
+          $flat_cats = array();
+          $flat_cats[0] = NO_CATEGORY;
+          foreach($cats AS $catidx => $catdata) {
+              $flat_cats[$catdata['categoryid']] = str_repeat(' ', $catdata['depth']*2) . serendipity_specialchars($catdata['category_name']);
+          }
+          return $flat_cats;
+        }
     }
     return $ret;
 }