[Icons] Add palette-aware themed icon engine

A QPixmap snapshot cannot follow palette changes, so expose a QIcon whose
engine resolves the `currentColor` sentinel from the live palette on every
pixmap request. Icons stay legible across light/dark switches without
recreating actions or buttons.

- Resolve the asset, its bytes and its intrinsic size once, and drop them on
  ThemeManager::themeChanged/paletteChanged. A repaint does no file I/O and
  no theme.cfg parse; the palette color stays part of the render cache key.
- Centre the glyph on QRectF(rect).center(): QRect::center() floors and uses
  right = left + width - 1, which shifted every 16x16 item-view glyph up and
  left by a logical pixel and clipped its last row and column.
- Take a raster override's intrinsic size from QImageReader instead of the
  24x24 SVG default, so a non-square override reports the box it draws.
- Set the device pixel ratio before inserting into the render cache, so a
  cache hit no longer detaches the pixmap on every paint.

oracle/CMakeLists.txt

@@ -31,6 +31,7 @@ set(oracle_SOURCES
     ../cockatrice/src/client/network/update/client/release_channel.cpp
     ../cockatrice/src/interface/pixel_map_generator.cpp
     ../cockatrice/src/interface/theme_config.cpp
+    ../cockatrice/src/interface/theme_icon.cpp
     ../cockatrice/src/interface/theme_manager.cpp
     ${VERSION_STRING_CPP}
 )