Add "added" timestamp to song database

- added is set to current time, if a new song is added to the database.
- GetAdded falls back to mtime.

Code for proxy plugin is missing, this needs a patch for libmpdclient.

closes #378

src/db/VHelper.cxx

@@ -60,6 +60,13 @@ DatabaseVisitorHelper::Commit()
 						 ? a.GetLastModified() > b.GetLastModified()
 						 : a.GetLastModified() < b.GetLastModified();
 				 });
+	else if (sort == TagType(SORT_TAG_ADDED))
+		std::stable_sort(songs.begin(), songs.end(),
+				 [descending](const DetachedSong &a, const DetachedSong &b){
+					 return descending
+						 ? a.GetAdded() > b.GetAdded()
+						 : a.GetAdded() < b.GetAdded();
+				 });
 	else
 		std::stable_sort(songs.begin(), songs.end(),
 				 [sort, descending](const DetachedSong &a,

src/db/plugins/simple/Song.cxx

@@ -19,6 +19,7 @@ Song::Song(DetachedSong &&other, Directory &_parent) noexcept
 	 filename(other.GetURI()),
 	 tag(std::move(other.WritableTag())),
 	 mtime(other.GetLastModified()),
+	 added(other.GetAdded()),
 	 start_time(other.GetStartTime()),
 	 end_time(other.GetEndTime()),
 	 audio_format(other.GetAudioFormat())
@@ -117,6 +118,9 @@ Song::Export() const noexcept
 	dest.mtime = IsNegative(mtime) && target_song != nullptr
 		? target_song->mtime
 		: mtime;
+	dest.added = IsNegative(added) && target_song != nullptr
+		? target_song->added
+		: added;
 	dest.start_time = start_time.IsZero() && target_song != nullptr
 		? target_song->start_time
 		: start_time;

src/db/plugins/simple/Song.hxx

@@ -56,6 +56,13 @@ struct Song : IntrusiveListHook<> {
 	std::chrono::system_clock::time_point mtime =
 		std::chrono::system_clock::time_point::min();
 
+	/**
+	 * The time stamp when the file was added. A negative
+	 * value means that this is unknown/unavailable.
+	 */
+	std::chrono::system_clock::time_point added =
+		std::chrono::system_clock::time_point::min();
+
 	/**
 	 * Start of this sub-song within the file.
 	 */

src/db/update/UpdateSong.cxx

@@ -50,6 +50,7 @@ try {
 		}
 
 		new_song->mark = true;
+		new_song->added = std::chrono::system_clock::now();
 
 		{
 			const ScopeDatabaseLock protect;