[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.cpp

@@ -1,6 +1,7 @@
 #include "games_model.h"
 
 #include "../../../client/settings/cache_settings.h"
+#include "../../theme_icon.h"
 #include "../interface/pixel_map_generator.h"
 #include "libcockatrice/protocol/pb/serverinfo_user.pb.h"
 #include "user/user_list_proxy.h"
@@ -188,7 +189,10 @@ QVariant GamesModel::data(const QModelIndex &index, int role) const
                     return result.join(", ");
                 }
                 case Qt::DecorationRole: {
-                    return gameentry.with_password() ? QIcon(LockPixmapGenerator::generatePixmap(13)) : QVariant();
+                    // The themed icon follows the palette, so the lock also
+                    // switches to HighlightedText on a selected row, and the
+                    // item view paints it at the device pixel ratio.
+                    return gameentry.with_password() ? loadThemedIcon(QStringLiteral("icons/lock")) : QVariant();
                     case Qt::TextAlignmentRole:
                         return Qt::AlignLeft;
                     default: