Cleanup: get rid of the variadic parameter of ContextMgr.Push

Apparently this was an attempt at working around go's lack of default arguments,
but it's very unidiomatic and a bit confusing. Make it a normal parameter
instead, so all clients have to pass it explicitly.

pkg/gui/view_helpers.go

@@ -72,7 +72,7 @@ func (gui *Gui) onViewTabClick(windowName string, tabIndex int) error {
 		return nil
 	}
 
-	gui.c.Context().Push(context)
+	gui.c.Context().Push(context, types.OnFocusOpts{})
 	return nil
 }