LibGUI: Add a simple GWidget class registry/factory

You can now register a GWidget subclass with REGISTER_GWIDGET(class)
and it will be available for factory construction through the new
GWidgetClassRegistration interface.

To obtain a GWidgetClassRegistration for a given class name, you call
GWidgetClassRegistration::find(class_name). You can also iterate over
all the registered classes using GWCR::for_each(callback).

This will be very useful for implementing a proper GUI designer, and
also in the future for things like script bindings.

NOTE: All of the registrations are done in GWidget.cpp at the moment
since I ran into trouble with the fricken linker pruning the global
constructors this mechanism relies on. :^)

Libraries/LibGUI/GTextEditor.cpp

@@ -15,6 +15,11 @@
 
 //#define DEBUG_GTEXTEDITOR
 
+GTextEditor::GTextEditor(GWidget* parent)
+    : GTextEditor(Type::MultiLine, parent)
+{
+}
+
 GTextEditor::GTextEditor(Type type, GWidget* parent)
     : GScrollableWidget(parent)
     , m_type(type)