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.cpp

@@ -9,6 +9,8 @@
 #include <LibGfx/Font.h>
 #include <LibGfx/FontDatabase.h>
 
+namespace Solitaire {
+
 static const NonnullRefPtr<Gfx::CharacterBitmap> s_diamond = Gfx::CharacterBitmap::create_from_ascii(
     "    #    "
     "   ###   "
@@ -155,3 +157,5 @@ void Card::clear_and_draw(GUI::Painter& painter, const Color& background_color)
     draw(painter);
     save_old_position();
 }
+
+}