cache: save disk cache index on a regular interval in case of crashing

DiskManager tries to keep track of all files made across sessions in an
index, but the index was only saved on close. This meant if the program
crashed (not an uncommon occurrence at the moment), it would "forget" about
any files it had made that session.

app/config/config.cpp

@@ -67,6 +67,7 @@ void Config::SetDefaults()
   SetEntryInternal(QStringLiteral("HoverFocus"), NodeParam::kBoolean, false);
   SetEntryInternal(QStringLiteral("AudioScrubbing"), NodeParam::kBoolean, true);
   SetEntryInternal(QStringLiteral("AutorecoveryInterval"), NodeParam::kInt, 1);
+  SetEntryInternal(QStringLiteral("DiskCacheSaveInterval"), NodeParam::kInt, 10000);
   SetEntryInternal(QStringLiteral("Language"), NodeParam::kString, QLocale::system().name());
   SetEntryInternal(QStringLiteral("ScrollZooms"), NodeParam::kBoolean, false);
   SetEntryInternal(QStringLiteral("EnableSeekToImport"), NodeParam::kBoolean, false);

app/render/diskmanager.cpp

@@ -189,6 +189,10 @@ DiskCacheFolder::DiskCacheFolder(const QString &path, QObject *parent) :
   QObject(parent)
 {
   SetPath(path);
+
+  save_timer_.setInterval(Config::Current()[QStringLiteral("DiskCacheSaveInterval")].toInt());
+  connect(&save_timer_, &QTimer::timeout, this, &DiskCacheFolder::SaveDiskCacheIndex);
+  save_timer_.start();
 }
 
 DiskCacheFolder::~DiskCacheFolder()
@@ -335,6 +339,11 @@ void DiskCacheFolder::CloseCacheFolder()
   }
 
   // Save current cache index
+  SaveDiskCacheIndex();
+}
+
+void DiskCacheFolder::SaveDiskCacheIndex()
+{
   QFile cache_index_file(index_path_);
 
   if (cache_index_file.open(QFile::WriteOnly)) {

app/render/diskmanager.h

@@ -24,6 +24,7 @@
 #include <QMap>
 #include <QMutex>
 #include <QObject>
+#include <QTimer>
 
 #include "common/define.h"
 #include "project/project.h"
@@ -97,6 +98,11 @@ private:
 
   bool clear_on_close_;
 
+  QTimer save_timer_;
+
+private slots:
+  void SaveDiskCacheIndex();
+
 };
 
 class DiskManager : public QObject