Cache icons in TorrentContentModel

This commit caches icons in TorrentContentModel in the same way they are
cached in TorrentModel.

This commit should improve performance when user navigating through
torrent list using up/down keys. A scrolling through all the list
(276 torrents) took:

    Total wall time:                            18.813s
    Total CPU time:                              3.210s
    IconProvider::generateDifferentSizes():      0.170s

IconProvider::generateDifferentSizes is 5th most hottest function on
this use case.

src/torrentcontentmodel.cpp

@@ -37,6 +37,18 @@
 #include "torrentcontentmodelfile.h"
 #include <QDir>
 
+namespace {
+QIcon get_directory_icon() {
+  static QIcon cached = IconProvider::instance()->getIcon("inode-directory");
+  return cached;
+}
+
+QIcon get_file_icon() {
+  static QIcon cached = IconProvider::instance()->getIcon("text-plain");
+  return cached;
+}
+}
+
 TorrentContentModel::TorrentContentModel(QObject *parent):
   QAbstractItemModel(parent),
     m_rootItem(new TorrentContentModelFolder(QList<QVariant>() << tr("Name") << tr("Size")
@@ -169,9 +181,9 @@ QVariant TorrentContentModel::data(const QModelIndex& index, int role) const
   TorrentContentModelItem* item = static_cast<TorrentContentModelItem*>(index.internalPointer());
   if (index.column() == 0 && role == Qt::DecorationRole) {
     if (item->itemType() == TorrentContentModelItem::FolderType)
-      return IconProvider::instance()->getIcon("inode-directory");
+      return get_directory_icon();
     else
-      return IconProvider::instance()->getIcon("text-plain");
+      return get_file_icon();
   }
   if (index.column() == 0 && role == Qt::CheckStateRole) {
     if (item->data(TorrentContentModelItem::COL_PRIO).toInt() == prio::IGNORED)