Added placeholder text support for option menu widget

[amtzespinosa]

I needed this feature for one of my projects. It's a simple modification to the original file but I think it might be quite useful to make fully custom app GUIs. You can find more info about the mod in my repo: CustomTkinter Option Menu Placeholder Text Mod.

And you can see it in action in my project #passman.

Hope you find it interesting. Cheers!