LibCards: Add a CardGame base class

For now, the only feature of this is that it sets the background colour
from the `Games::Cards::BackgroundColor` config value. Later, other
card game configuration and shared behaviour can go here, to save
duplicating it in each game.

Userland/Libraries/LibCards/CMakeLists.txt

@@ -1,7 +1,8 @@
 set(SOURCES
     Card.cpp
+    CardGame.cpp
     CardStack.cpp
 )
 
 serenity_lib(LibCards cards)
-target_link_libraries(LibCards LibC LibCore)
+target_link_libraries(LibCards LibC LibCore LibConfig LibGUI)