LibGUI: Move REGISTER_WIDGET(GUI, TabWidget) to Widget.cpp

All the other (GUI) registrations happen here, so it's confusing to have
just one elsewhere ("wait... is TabWidget not registered?!").

Libraries/LibGUI/TabWidget.cpp

@@ -35,8 +35,6 @@
 #include <LibGfx/Palette.h>
 #include <LibGfx/StylePainter.h>
 
-REGISTER_WIDGET(GUI, TabWidget)
-
 namespace GUI {
 
 TabWidget::TabWidget()

Libraries/LibGUI/Widget.cpp

@@ -47,6 +47,7 @@
 #include <LibGUI/SpinBox.h>
 #include <LibGUI/Splitter.h>
 #include <LibGUI/StatusBar.h>
+#include <LibGUI/TabWidget.h>
 #include <LibGUI/TextBox.h>
 #include <LibGUI/ToolBar.h>
 #include <LibGUI/ToolBarContainer.h>
@@ -75,6 +76,7 @@ REGISTER_WIDGET(GUI, ScrollBar)
 REGISTER_WIDGET(GUI, Slider)
 REGISTER_WIDGET(GUI, SpinBox)
 REGISTER_WIDGET(GUI, StatusBar)
+REGISTER_WIDGET(GUI, TabWidget)
 REGISTER_WIDGET(GUI, TextBox)
 REGISTER_WIDGET(GUI, TextEditor)
 REGISTER_WIDGET(GUI, ToolBar)