Lua: Make Layout-derived classes directly show-able

Change-Id: I2bea8187626c239b5b2e9ddb32d291db97ddc9ac
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/libs/utils/layoutbuilder.h

@@ -156,6 +156,7 @@ public:
     void fieldGrowthPolicy(int policy);
 
     QWidget *emerge() const;
+    void show() const;
 
     QFormLayout *asForm();
     QGridLayout *asGrid();