QmlJS: Speed up ValueOwner construction.

* Don't build all default values (including the global object)
  separately for each ValueOwner instance.
* Instead, keep all global, immutable values in a single, shared
  instance.

While refactoring, some cases where we *modified* the global object had
to be removed:

* C++ context properties no longer get injected into the global object,
  instead they now have their own scope just above the global one.
* The Qt object's prototype no longer gets modified in Link. Instead,
  it's now a reference to the "Qt" object provided in a qmltypes file.
* The whole concept of a function 'Activation' that could potentially
  affect the global object was removed.

Change-Id: Id382faf965efa747fcc7a9b0bc2c90429d84d61b
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>

src/libs/qmljs/qmljsevaluate.cpp

@@ -273,13 +273,13 @@ bool Evaluate::visit(AST::NumericLiteral *)
 
 bool Evaluate::visit(AST::RegExpLiteral *)
 {
-    _result = _valueOwner->regexpCtor()->construct();
+    _result = _valueOwner->regexpCtor()->returnValue();
     return false;
 }
 
 bool Evaluate::visit(AST::ArrayLiteral *)
 {
-    _result = _valueOwner->arrayCtor()->construct();
+    _result = _valueOwner->arrayCtor()->returnValue();
     return false;
 }
 
@@ -347,7 +347,7 @@ bool Evaluate::visit(AST::FieldMemberExpression *ast)
 bool Evaluate::visit(AST::NewMemberExpression *ast)
 {
     if (const FunctionValue *ctor = value_cast<FunctionValue>(value(ast->base))) {
-        _result = ctor->construct();
+        _result = ctor->returnValue();
     }
     return false;
 }
@@ -355,7 +355,7 @@ bool Evaluate::visit(AST::NewMemberExpression *ast)
 bool Evaluate::visit(AST::NewExpression *ast)
 {
     if (const FunctionValue *ctor = value_cast<FunctionValue>(value(ast->expression))) {
-        _result = ctor->construct();
+        _result = ctor->returnValue();
     }
     return false;
 }