directory: added directory_is_empty()

directory_is_empty() is a tiny inline function which determine if a
directory has any child objects (sub directories or songs).

src/directory.h

@@ -23,6 +23,8 @@
 #include "songvec.h"
 #include "list.h"
 
+#include <stdbool.h>
+
 struct dirvec {
 	struct _Directory **base;
 	size_t nr;
@@ -53,6 +55,12 @@ newDirectory(const char *dirname, Directory * parent);
 void
 freeDirectory(Directory * directory);
 
+static inline bool
+directory_is_empty(Directory *directory)
+{
+	return directory->children.nr == 0 && directory->songs.nr == 0;
+}
+
 Directory *
 getDirectory(const char *name);