Select Master Channel dialogue: make it work for multiple cards

If the combo box is needed the mixer ID was not being set in its
item data, so createPageByID() would not find the mixer.

gui/dialogselectmaster.cpp

@@ -57,16 +57,19 @@ void DialogSelectMaster::createWidgets(const Mixer *mixer)
     QVBoxLayout *layout = new QVBoxLayout(mainFrame);
 
     const QList<Mixer *> &mixers = Mixer::mixers();	// list of all mixers present
-    int mixerIndex = 0;					// index of specified 'mixer'
 
     if (mixers.count()>1)
     {
+        int mixerIndex = 0;				// index of selected mixer
+
+        // TODO: should "Playback Streams" (ALSA) be shown here?
+        // There is no meaningful concept of a master channel for that.
+
         // More than one Mixer => show Combo-Box to select Mixer
         // Mixer widget line
-        QHBoxLayout* mixerNameLayout = new QHBoxLayout();
+        QHBoxLayout *mixerNameLayout = new QHBoxLayout();
         layout->addLayout( mixerNameLayout );
         mixerNameLayout->setContentsMargins(0, 0, 0, 0);
-        mixerNameLayout->setSpacing(DialogBase::horizontalSpacing());
     
         QLabel *qlbl = new QLabel( i18n("Current mixer:"), mainFrame );
         mixerNameLayout->addWidget(qlbl);
@@ -75,14 +78,13 @@ void DialogSelectMaster::createWidgets(const Mixer *mixer)
         m_cMixer = new QComboBox(mainFrame);
         m_cMixer->setObjectName( QLatin1String( "mixerCombo" ) );
         m_cMixer->setFixedHeight(m_cMixer->sizeHint().height());
-        connect( m_cMixer, SIGNAL(activated(int)), this, SLOT(createPageByID(int)) );
+        connect(m_cMixer, QOverload<int>::of(&QComboBox::activated), this, &DialogSelectMaster::createPageByID);
 
         for (int i = 0; i<mixers.count(); ++i)
         {
             const Mixer *m = mixers[i];
-            const QString name = m->readableName();
-            m_cMixer->addItem(QIcon::fromTheme(m->iconName()), name);
-            if (name==mixer->readableName()) mixerIndex = i;
+            m_cMixer->addItem(QIcon::fromTheme(m->iconName()), m->readableName(), m->id());
+            if (m->id()==mixer->id()) mixerIndex = i;
          }
 
         // Select the specified 'mixer' as the current item in the combo box.
@@ -117,6 +119,8 @@ void DialogSelectMaster::createWidgets(const Mixer *mixer)
  */
 void DialogSelectMaster::createPageByID(int mixerId)
 {
+    // TODO: would Mixer::mixers().value(mixerId) work just as well?
+    // TODO: mixerId should really be named mixerIndex!
     const QString mixer_id = m_cMixer->itemData(mixerId).toString();
     const Mixer *mixer = Mixer::findMixer(mixer_id);
     if (mixer!=nullptr) createPage(mixer);
@@ -154,24 +158,22 @@ void DialogSelectMaster::createPage(const Mixer *mixer)
 	layout->addWidget(m_channelSelector);
 
         const MixSet &mixset = mixer->getMixSet();
-        const MixSet &mset = const_cast<MixSet &>(mixset);
-
 	const MasterControl mc = mixer->getGlobalMasterPreferred(false);
 	QString masterKey = mc.getControl();
-	if (!masterKey.isEmpty() && !mset.get(masterKey))
+	if (!masterKey.isEmpty() && !mixset.get(masterKey))
 	{
 		const shared_ptr<MixDevice> master = mixer->getLocalMasterMD();
 		if (master.get()!=nullptr) masterKey = master->id();
 	}
 
 	int msetCount = 0;
-	for (int i = 0; i < mset.count(); ++i)
+	for (int i = 0; i<mixset.count(); ++i)
         {
-            const shared_ptr<MixDevice> md = mset[i];
+            const shared_ptr<MixDevice> md = mixset[i];
             if (md->playbackVolume().hasVolume()) ++msetCount;
         }
 
-	if (msetCount > 0 && !mixer->isDynamic())
+	if (msetCount>0 && !mixer->isDynamic())
 	{
             QString mdName = i18n("Automatic (%1 recommendation)", mixer->getDriverName());
             auto *item = new QListWidgetItem(QIcon::fromTheme("audio-volume-high"), mdName, m_channelSelector);
@@ -181,9 +183,9 @@ void DialogSelectMaster::createPage(const Mixer *mixer)
 
 	// Populate the list view with the MixDevice's having a playback volume
         // (excludes pure capture controls and pure enum's)
-	for (int i = 0; i < mset.count(); ++i)
+	for (int i = 0; i<mixset.count(); ++i)
         {
-            const shared_ptr<MixDevice> md = mset[i];
+            const shared_ptr<MixDevice> md = mixset[i];
             if (md->playbackVolume().hasVolume())
             {
                 const QString mdName = md->readableName();