Get rid of the "root widget" concept in WindowManager.

Instead just create a GraphicsBitmap wrapper around the display framebuffer
and teach Painter how to draw directly into a GraphicsBitmap.

Kernel/Makefile

@@ -55,7 +55,6 @@ WIDGETS_OBJS = \
     ../Widgets/Color.o \
     ../Widgets/CharacterBitmap.o \
     ../Widgets/EventLoop.o \
-    ../Widgets/RootWidget.o \
     ../Widgets/Label.o \
     ../Widgets/Button.o \
     ../Widgets/MsgBox.o \

Kernel/WindowComposer.cpp

@@ -3,7 +3,6 @@
 #include <Widgets/Font.h>
 #include <Widgets/FrameBuffer.h>
 #include <Widgets/WindowManager.h>
-#include <Widgets/RootWidget.h>
 #include <Widgets/EventLoop.h>
 #include <Widgets/MsgBox.h>
 #include <Widgets/TextBox.h>
@@ -26,11 +25,8 @@ void WindowComposer_main()
     dbgprintf("Screen is %ux%ux%ubpp\n", info.width, info.height, info.bpp);
 
     FrameBuffer framebuffer((dword*)info.framebuffer, info.width, info.height);
-    RootWidget rw;
     EventLoop loop;
 
-    WindowManager::the().setRootWidget(&rw);
-
     MsgBox(nullptr, "Serenity Operating System");
 
     {