Replace _IRR_OVERRIDE_ macro with override keyword

The commit also establishes a precedent of leaving off the `virtual`
keyword in overrides. Although not strictly necessary, I believe this is
good for readability because it makes it clear it is an override and not
a pure virtual function, and it helps keep line lengths shorter. We
should move towards eliminating the macro altogether, but the definition
has been left in with a note on deprecation so that in-progress work
will not suffer merge conflicts.

include/IGUIElement.h

@@ -545,7 +545,7 @@ public:
 
 
 	//! Called if an event happened.
-	virtual bool OnEvent(const SEvent& event) _IRR_OVERRIDE_
+	bool OnEvent(const SEvent& event) override
 	{
 		return Parent ? Parent->OnEvent(event) : false;
 	}