New context with albums and lyrics +++ much more

* Added new lyrics provider with lyrics from AudD and API Seeds
* New improved context widget with albums and lyrics
* Fixed playing and context widget getting stuck in play mode when there was an error
* Changed icons for artists in collection, tidal and cover manager
* Removed "search" icon from "Search automatically" checkbox (right click) that looked ugly
* Removed some unused widgets from the src/widgets directory
* Fixed initial size of window and side panel
* Fixed saving window size correctly

src/lyrics/lyricsproviders.cpp

@@ -0,0 +1,76 @@
+/*
+ * Strawberry Music Player
+ * Copyright 2018, Jonas Kvinge <jonas@jkvinge.net>
+ *
+ * Strawberry is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Strawberry is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Strawberry.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include "config.h"
+
+#include <QObject>
+#include <QMutex>
+#include <QString>
+#include <QtDebug>
+
+#include "core/application.h"
+#include "core/logging.h"
+#include "lyricsprovider.h"
+#include "lyricsproviders.h"
+#include "lyricsfetcher.h"
+
+LyricsProviders::LyricsProviders(QObject *parent) : QObject(parent) {}
+
+void LyricsProviders::AddProvider(LyricsProvider *provider) {
+
+  {
+    QMutexLocker locker(&mutex_);
+    lyrics_providers_.insert(provider, provider->name());
+    connect(provider, SIGNAL(destroyed()), SLOT(ProviderDestroyed()));
+  }
+
+  qLog(Debug) << "Registered lyrics provider" << provider->name();
+
+}
+
+void LyricsProviders::RemoveProvider(LyricsProvider *provider) {
+
+  if (!provider) return;
+
+  // It's not safe to dereference provider at this point because it might have already been destroyed.
+
+  QString name;
+
+  {
+    QMutexLocker locker(&mutex_);
+    name = lyrics_providers_.take(provider);
+  }
+
+  if (name.isNull()) {
+    qLog(Debug) << "Tried to remove a lyrics provider that was not registered";
+  }
+  else {
+    qLog(Debug) << "Unregistered lyrics provider" << name;
+  }
+
+}
+
+void LyricsProviders::ProviderDestroyed() {
+
+  LyricsProvider *provider = static_cast<LyricsProvider*>(sender());
+  RemoveProvider(provider);
+
+}
+
+int LyricsProviders::NextId() { return next_id_.fetchAndAddRelaxed(1); }