QPlatformTheme: added TabButtonFont and GroupBoxTitleFont

Which is needed by Qt Labs Controls.

Change-Id: I2f91a1cbe8218170f894189b56f3fef77207eaec
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>

src/gui/kernel/qplatformtheme.h

@@ -164,6 +164,8 @@ public:
         SmallFont,
         MiniFont,
         FixedFont,
+        GroupBoxTitleFont,
+        TabButtonFont,
         NFonts
     };