TextEditor: Implement highlighting of function definitions

This allows users to style function names at their definitions.

Once set, the XML-style token "FunctionDefinition" will
highlight all function definitions: the style option is a
mixin to Function and Virtual Function.

TEST=Default themes and locally hacked themes that lack Function,
FunctionDefinition, Declaration-styling look as they did before
this patch.

Requires Clang.

Task-number: QTCREATORBUG-16625
Change-Id: I49d8e401211bdf28ff74699feac16fe98f6d64ce
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

tests/unit/unittest/data/highlightingmarks.cpp

@@ -582,3 +582,9 @@ void f32()
 
 namespace N { template <typename T> void SizeIs(); }
 using N::SizeIs;
+
+void BaseClass::VirtualFunction() {}
+
+class WithVirtualFunctionDefined {
+  virtual void VirtualFunctionDefinition() {};
+};