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/GSlider.cpp

@@ -2,6 +2,11 @@
 #include <LibGUI/GPainter.h>
 #include <LibGUI/GSlider.h>
 
+GSlider::GSlider(GWidget* parent)
+    : GSlider(Orientation::Horizontal, parent)
+{
+}
+
 GSlider::GSlider(Orientation orientation, GWidget* parent)
     : GWidget(parent)
     , m_orientation(orientation)