Don't recalculate the song length for every added TCO while loading (#5236)

Don't make LMMS calculate the song length for every added TCO when a new project is created or a project is loaded. Instead do it only once afterwards. This is accomplished by preventing any calculations in `Song::updateLength` if a song is currently loaded. `Song::updateLength` is then called immediately after the loading flag has been set to `false` in both cases.

---------

Co-authored-by: IanCaio <iancaio_dev@hotmail.com>

src/core/Song.cpp

@@ -562,6 +562,8 @@ void Song::playMidiClip( const MidiClip* midiClipToPlay, bool loop )
 
 void Song::updateLength()
 {
+	if (m_loadingProject) { return; }
+
 	m_length = 0;
 	m_tracksMutex.lockForRead();
 	for (auto track : tracks())
@@ -963,7 +965,7 @@ void Song::createNewProject()
 	QCoreApplication::instance()->processEvents();
 
 	m_loadingProject = false;
-
+	updateLength();
 	Engine::patternStore()->updateAfterTrackAdd();
 
 	Engine::projectJournal()->setJournalling( true );
@@ -1206,6 +1208,7 @@ void Song::loadProject( const QString & fileName )
 	}
 
 	m_loadingProject = false;
+	updateLength();
 	setModified(false);
 	m_loadOnLaunch = false;
 }