Change LibraryDirectoryModel to take shared backend pointer.

This is mainly for parity amongst the library model classes.

src/library/library.cpp

@@ -57,7 +57,7 @@ Library::Library(Application* app, QObject* parent)
   using smart_playlists::SearchTerm;
 
   model_ = new LibraryModel(backend_, app_, this);
-  dir_model_ = new LibraryDirectoryModel(backend_.get(), this);
+  dir_model_ = new LibraryDirectoryModel(backend_, this);
   model_->set_show_smart_playlists(true);
   model_->set_default_smart_playlists(
       LibraryModel::DefaultGenerators()

src/library/librarydirectorymodel.cpp

@@ -23,14 +23,16 @@
 #include "core/utilities.h"
 #include "ui/iconloader.h"
 
-LibraryDirectoryModel::LibraryDirectoryModel(LibraryBackend* backend,
-                                             QObject* parent)
+LibraryDirectoryModel::LibraryDirectoryModel(
+    std::shared_ptr<LibraryBackend> backend, QObject* parent)
     : QStandardItemModel(parent),
       dir_icon_(IconLoader::Load("document-open-folder", IconLoader::Base)),
       backend_(backend) {
-  connect(backend_, SIGNAL(DirectoryDiscovered(Directory, SubdirectoryList)),
+  connect(backend_.get(),
+          SIGNAL(DirectoryDiscovered(Directory, SubdirectoryList)),
           SLOT(DirectoryDiscovered(Directory)));
-  connect(backend_, SIGNAL(DirectoryDeleted(int)), SLOT(DirectoryDeleted(int)));
+  connect(backend_.get(), SIGNAL(DirectoryDeleted(int)),
+          SLOT(DirectoryDeleted(int)));
 }
 
 LibraryDirectoryModel::~LibraryDirectoryModel() {}

src/library/librarydirectorymodel.h

@@ -32,7 +32,8 @@ class LibraryDirectoryModel : public QStandardItemModel {
   Q_OBJECT
 
  public:
-  LibraryDirectoryModel(LibraryBackend* backend, QObject* parent = nullptr);
+  LibraryDirectoryModel(std::shared_ptr<LibraryBackend> backend,
+                        QObject* parent = nullptr);
   ~LibraryDirectoryModel();
 
   // To be called by GUIs
@@ -50,7 +51,7 @@ class LibraryDirectoryModel : public QStandardItemModel {
   static const int kIdRole = Qt::UserRole + 1;
 
   QIcon dir_icon_;
-  LibraryBackend* backend_;
+  std::shared_ptr<LibraryBackend> backend_;
   QList<std::shared_ptr<MusicStorage> > storage_;
 };