Themes: Enable using the original Window color for toolbars

This patch adds a flag which enables the "fancy" side/tool/status bars
to use the original Window color from the application palette.

The flag is disabled for all themes by default.

Task-number: QTCREATORBUG-16331
Change-Id: I1433e865e1903012d521a2cdb7083f110086e455
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@theqtcompany.com>

src/libs/utils/stylehelper.cpp

@@ -114,6 +114,12 @@ QColor StyleHelper::m_requestedBaseColor;
 
 QColor StyleHelper::baseColor(bool lightColored)
 {
+    static const bool windowColorAsBase = creatorTheme()->flag(Theme::WindowColorAsBase);
+    if (windowColorAsBase) {
+        static const QColor windowColor = QApplication::palette().color(QPalette::Window);
+        return windowColor;
+    }
+
     if (!lightColored)
         return m_baseColor;
     else