Fix layoutFlags assignment in CreateLayoutItem method

[merlinz01]

These were switched, causing a horizontal separator to appear as a vertical one and presumably vice versa.

mainWindow, err := walk.NewMainWindowWithCfg(&walk.MainWindowCfg{})

...

mainLayout = walk.NewVBoxLayout()
mainWindow.SetLayout(mainLayout)

...

separator, _ := walk.NewHSeparator(w.mainWindow)

Before:

After:

[merlinz01]

On the other hand, maybe the horizontal/vertical is referring to the layout it goes in, e.g. a vertical separator is a (horizontal) line that separates a vertical layout.

I think it's clearer the other way though.