LibCore: Introduce a C_OBJECT macro.

This macro goes at the top of every CObject-derived class like so:

class SomeClass : public CObject {
    C_OBJECT(SomeClass)
public:
    ...

At the moment, all it does is create an override for the class_name() getter
but in the future this will be used to automatically insert member functions
into these classes.

Servers/WindowServer/WSWindowManager.h

@@ -42,6 +42,8 @@ enum class ResizeDirection {
 };
 
 class WSWindowManager : public CObject {
+    C_OBJECT(WSWindowManager)
+
     friend class WSCompositor;
     friend class WSWindowFrame;
     friend class WSWindowSwitcher;