LibGUI: Start adding an automatic widget layout system.

My needs are really quite simple, so I'm just going to add what I need
as I go along. The first thing I needed was a simple box layout with
widgets being able to say whether they prefer fixed or fill for both
their vertical and horizontal sizes.

I also made a simple GStatusBar so FileManager can show how many bytes
worth of files are in the current directory.

LibC/entry.cpp

@@ -38,3 +38,9 @@ epilogue:
     // Birger's birthday <3
     return 20150614;
 }
+
+extern "C" void __cxa_pure_virtual() NORETURN;
+extern "C" void __cxa_pure_virtual()
+{
+    ASSERT_NOT_REACHED();
+}