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.cpp

@@ -6,6 +6,8 @@
 
 #include "CardStack.h"
 
+namespace Solitaire {
+
 CardStack::CardStack()
     : m_position({ 0, 0 })
     , m_type(Invalid)
@@ -216,3 +218,5 @@ void CardStack::calculate_bounding_box()
 
     m_bounding_box.set_size(Card::width + width, Card::height + height);
 }
+
+}