Flat themes: Implement Diana's separators

The flat theme as defined by Diana has the motto:
"Remove unnecessary lines, avoid gradients".

This patch implemets that for the flat themes, while
leaving the default theme unchanged.

The only tweak that touches all styles is that
"Styled Separator" has for consitency now the same look
as standard toolbar separators.

Change-Id: I80ab0d9bf5ca20ab0daa57baf5a7e095a4a0c653
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>

src/libs/utils/stylehelper.cpp

@@ -145,6 +145,11 @@ QColor StyleHelper::borderColor(bool lightColored)
     return result;
 }
 
+QColor StyleHelper::separatorColor()
+{
+    return borderColor(false);
+}
+
 // We try to ensure that the actual color used are within
 // reasonalbe bounds while generating the actual baseColor
 // from the users request.