LibCards: Support highlighting cards of interest

For example, in Solitaire, when dragging a card around, it's common for
other implementations to highlight the card underneath the dragged card
if that other card is a valid drop target. This implementation will draw
a rounded rectangle within the edges of the highlighted card, using a
rudimentary complementary color of the board background color.

Userland/Libraries/LibCards/CardGame.cpp

@@ -126,5 +126,8 @@ void CardGame::set_background_color(Gfx::Color color)
     auto new_palette = palette();
     new_palette.set_color(Gfx::ColorRole::Background, color);
     set_palette(new_palette);
+
+    CardPainter::the().set_background_color(color);
 }
+
 }