Add a function to format X11 window handle from uintptr

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

The window handle needs to be formatted as a string from a uintptr. We can make life easier for developers by introducing a helper for this.

Describe the solution you'd like to see.

This is made manually in Fyne, we can bring the functionality into this package instead.