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/CardStack.h

@@ -9,6 +9,8 @@
 #include "Card.h"
 #include <AK/Vector.h>
 
+namespace Solitaire {
+
 class CardStack final {
 public:
     enum Type {
@@ -78,3 +80,5 @@ private:
     bool m_dirty { false };
     Gfx::IntRect m_base;
 };
+
+}