LibGUI: Put all classes in the GUI namespace and remove the leading G

This took me a moment. Welcome to the new world of GUI::Widget! :^)

Libraries/LibGUI/GCommand.cpp

@@ -26,6 +26,10 @@
 
 #include <LibGUI/GCommand.h>
 
-GCommand::~GCommand()
+namespace GUI {
+
+Command::~Command()
 {
 }
+
+}