refactor(Nexus): Remove static getCameraSource

On path towards removing Nexus singleton.

src/appmanager.cpp

@@ -363,7 +363,7 @@ int AppManager::run()
     nexus.setSettings(settings.get());
     nexus.setMessageBoxManager(messageBoxManager.get());
     nexus.setIpc(ipc.get());
-    auto& cameraSource = Nexus::getCameraSource();
+    auto& cameraSource = nexus.getCameraSource();
     // Autologin
     // TODO (kriby): Shift responsibility of linking views to model objects from nexus
     // Further: generate view instances separately (loginScreen, mainGUI, audio)

src/nexus.cpp

@@ -333,7 +333,7 @@ void Nexus::setParser(QCommandLineParser* parser_)
 
 CameraSource& Nexus::getCameraSource()
 {
-    return *getInstance().cameraSource;
+    return *cameraSource;
 }
 
 void Nexus::setMessageBoxManager(IMessageBoxManager* messageBoxManager_)

src/nexus.h

@@ -59,7 +59,7 @@ public:
     static Nexus& getInstance();
     static void destroyInstance();
     Profile* getProfile();
-    static CameraSource& getCameraSource();
+    CameraSource& getCameraSource();
     void registerIpcHandlers();
     bool handleToxSave(const QString& path);