Don't create temporary containers just to iterate over them

Stops temporary containers being created needlessly due to API misuse.
For example, it’s common for developers to assume QHash::values() and
QHash::keys() are free and abuse them, failing to realize their
implementation internally actually iterates the whole container, allocates
memory, and fills a new QList.

Added a removeIf generic algorithm, similar to std ones. We can't use std
algorithms with Qt dictionaries because Qt iterators have different
behavior from the std ones.

Found using clazy.

src/gui/categoryfiltermodel.cpp

@@ -405,7 +405,8 @@ void CategoryFilterModel::populate()
                                     , [](Torrent *torrent) { return torrent->category().isEmpty(); })));
 
     using Torrent = BitTorrent::TorrentHandle;
-    foreach (const QString &category, session->categories().keys()) {
+    for (auto i = session->categories().cbegin(); i != session->categories().cend(); ++i) {
+        const QString &category = i.key();
         if (m_isSubcategoriesEnabled) {
             CategoryModelItem *parent = m_rootItem;
             foreach (const QString &subcat, session->expandCategory(category)) {

src/gui/categoryfilterwidget.cpp

@@ -232,8 +232,10 @@ void CategoryFilterWidget::removeCategory()
 void CategoryFilterWidget::removeUnusedCategories()
 {
     auto session = BitTorrent::Session::instance();
-    foreach (const QString &category, session->categories().keys())
+    for (auto i = session->categories().cbegin(); i != session->categories().cend(); ++i) {
+        const QString &category = i.key();
         if (model()->data(static_cast<CategoryFilterProxyModel *>(model())->index(category), Qt::UserRole) == 0)
             session->removeCategory(category);
+    }
     updateGeometry();
 }

src/gui/search/pluginselectdlg.cpp

@@ -424,10 +424,10 @@ void PluginSelectDlg::checkForUpdatesFinished(const QHash<QString, PluginVersion
         return;
     }
 
-    foreach (const QString &pluginName, updateInfo.keys()) {
+    for (auto i = updateInfo.cbegin(); i != updateInfo.cend(); ++i) {
         startAsyncOp();
         m_pendingUpdates++;
-        m_pluginManager->updatePlugin(pluginName);
+        m_pluginManager->updatePlugin(i.key());
     }
 }
 

src/gui/transferlistfilterswidget.cpp

@@ -333,7 +333,8 @@ void TrackerFiltersList::setDownloadTrackerFavicon(bool value)
     m_downloadTrackerFavicon = value;
 
     if (m_downloadTrackerFavicon) {
-        foreach (const QString &tracker, m_trackers.keys()) {
+        for (auto i = m_trackers.cbegin(); i != m_trackers.cend(); ++i) {
+            const QString &tracker = i.key();
             if (!tracker.isEmpty())
                 downloadFavicon(QString("http://%1/favicon.ico").arg(tracker));
         }