Solitaire: Place files in Solitaire namespace and rename main widget

The purpose is to allow the Solitaire widget to be used in GML. The
macro to register a widget requires a namespace, so this moves all files
in the application to the Solitaire namespace. This also renames the
SolitaireWidget class to Game - this is to avoid the redundancy /
verbosity of typing "Solitaire::SolitaireWidget", and matches many other
games in Serenity (Breakout, 2048, etc.).

Userland/Games/Solitaire/CMakeLists.txt

@@ -1,8 +1,8 @@
 set(SOURCES
     Card.cpp
     CardStack.cpp
+    Game.cpp
     main.cpp
-    SolitaireWidget.cpp
 )
 
 serenity_app(Solitaire ICON app-solitaire)