[Icons] Render lock and dropdown glyphs through the icon engine

Both generators rendered the sentinel SVG to a fixed height x height pixmap
at DPR 1. The pre-migration path rendered larger and let QIcon scale it, so
on a 2x display the games-list lock (13px) and the banner dropdown arrow
(24px) came back crisp; the new path hands the widget a DPR-1 bitmap that
gets upscaled and blurred.

- The games list asks the themed engine for `icons/lock` directly, so the
  item view paints it at the device pixel ratio and the lock also follows
  HighlightedText on a selected row.
- The dropdown goes through `loadThemedIcon(...).pixmap(size, dpr)`, which
  keeps the glyph's aspect ratio and renders at the requested ratio.
- LockPixmapGenerator has no consumer left, so it and its cache are gone.

cockatrice/src/interface/widgets/server/games_model.h

@@ -5,6 +5,7 @@
 #include "game_type_map.h"
 
 #include <QAbstractTableModel>
+#include <QIcon>
 #include <QList>
 #include <QMap>
 #include <QModelIndex>
@@ -32,6 +33,7 @@ private:
     QList<ServerInfo_Game> gameList;  /**< List of games currently displayed. */
     QMap<int, QString> rooms;         /**< Map of room IDs to room names. */
     QMap<int, GameTypeMap> gameTypes; /**< Map of room IDs to available game types. */
+    QIcon lockIcon;                   /**< Themed lock for password-protected games. */
 
     static const int NUM_COLS = 8; /**< Number of columns in the table. */