OverlayManager: fix unititialized hideTab variable

The hideTab is of type bool, which is primitive type and is thus unitialized if new
instance of the class is created. The exception would be to for static
instance of the class, but that is not the case (only the pointer to the instance is static).

It was originaly initialized, but the initialization was removed in
9d5390e.

src/Gui/OverlayManager.cpp

@@ -136,7 +136,7 @@ class OverlayStyleSheet: public ParameterGrp::ObserverType {
 
     ParameterGrp::handle handle;
     QString activeStyleSheet;
-    bool hideTab;
+    bool hideTab = false;
 
 private:
     QString detectOverlayStyleSheetFileName() const {