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/Card.h

@@ -13,6 +13,8 @@
 #include <LibGfx/Rect.h>
 #include <ctype.h>
 
+namespace Solitaire {
+
 class Card final : public Core::Object {
     C_OBJECT(Card)
 public:
@@ -63,3 +65,5 @@ private:
     bool m_moving { false };
     bool m_upside_down { false };
 };
+
+}