Avoid db commits during startup

When starting clementine, each playlist is loaded in turn.
When loading a playlist, the new tab order is committed to the db,
but we don't need to do that here because we know that once all the
playlists are loaded the tab order will be the same as it was initially.

This speeds startup substantially.
kstartperf:
Before: 3.5s
After: 1.5s

src/playlist/playlisttabbar.cpp

@@ -44,6 +44,7 @@ PlaylistTabBar::PlaylistTabBar(QWidget* parent)
       menu_(new QMenu(this)),
       menu_index_(-1),
       suppress_current_changed_(false),
+      initialized_(false),
       rename_editor_(new RenameTabLineEdit(this)) {
   setAcceptDrops(true);
   setElideMode(Qt::ElideRight);
@@ -79,6 +80,16 @@ void PlaylistTabBar::SetManager(PlaylistManager* manager) {
   manager_ = manager;
   connect(manager_, SIGNAL(PlaylistFavorited(int, bool)),
           SLOT(PlaylistFavoritedSlot(int, bool)));
+  connect(manager_, SIGNAL(PlaylistManagerInitialized()), this,
+          SLOT(PlaylistManagerInitialized()));
+}
+
+void PlaylistTabBar::PlaylistManagerInitialized() {
+  // Signal that we are done loading and thus further changes should be
+  // committed to the db.
+  initialized_ = true;
+  disconnect(manager_, SIGNAL(PlaylistManagerInitialized()), this,
+             SLOT(PlaylistManagerInitialized()));
 }
 
 void PlaylistTabBar::contextMenuEvent(QContextMenuEvent* e) {
@@ -281,6 +292,7 @@ void PlaylistTabBar::InsertTab(int id, int index, const QString& text,
   insertTab(index, text);
   setTabData(index, id);
   setTabToolTip(index, text);
+
   FavoriteWidget* widget = new FavoriteWidget(id, favorite);
   widget->setToolTip(
       tr("Click here to favorite this playlist so it will be saved and remain "
@@ -291,14 +303,19 @@ void PlaylistTabBar::InsertTab(int id, int index, const QString& text,
   setTabButton(index, QTabBar::LeftSide, widget);
   suppress_current_changed_ = false;
 
-  if (currentIndex() == index) emit CurrentIdChanged(id);
+  // If we are still starting up, we don't need to do this, as the
+  // tab ordering after startup will be the same as was already in the db.
+  if (initialized_) {
+    if (currentIndex() == index) emit CurrentIdChanged(id);
 
-  // Update playlist tab order/visibility
-  TabMoved();
+    // Update playlist tab order/visibility
+    TabMoved();
+  }
 }
 
 void PlaylistTabBar::TabMoved() {
   QList<int> ids;
+
   for (int i = 0; i < count(); ++i) {
     ids << tabData(i).toInt();
   }

src/playlist/playlisttabbar.h

@@ -82,6 +82,8 @@ signals:
   // Used when playlist's favorite flag isn't changed from the favorite widget
   // (e.g. from the playlistlistcontainer): will update the favorite widget
   void PlaylistFavoritedSlot(int id, bool favorite);
+  // Used to signal that the playlist manager is done starting up
+  void PlaylistManagerInitialized();
   void TabMoved();
   void Save();
 
@@ -99,6 +101,7 @@ signals:
   int drag_hover_tab_;
 
   bool suppress_current_changed_;
+  bool initialized_;
 
   // Editor for inline renaming
   RenameTabLineEdit* rename_editor_;