QPlatformTheme: added EditorFont

Which is needed by Qt Labs Controls.

Task-number: QTBUG-51203
Change-Id: If5f39d59c5c88de37c9b034b784c38b976759439
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>

src/gui/kernel/qplatformtheme.h

@@ -172,6 +172,7 @@ public:
         FixedFont,
         GroupBoxTitleFont,
         TabButtonFont,
+        EditorFont,
         NFonts
     };