Decouple clipboard from fyne.Window

[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 clipboard in GLFW was previously tied to the window. This is no longer the case so I think it seems strange to have a window.Clipboard() method when clipboards generally are per app or per system and not per window.

See #4417 for step one.

Is it possible to construct a solution with the existing API?

Yeah. We can move the clipboard struct anywhere we'd like.

Describe the solution you'd like to see.

Add a app.Clipboard() method and deprecate the one on the window.

[andydotxyz]

This will make some system tray people happy as well, because we can then use clipboard without having any windows created.

[Jacalz]

Cool. I'll assign myself and look into implementing this