Allow copy/paste of image data between qubes using global clipboard

[Atrate]

The problem you're addressing (if any)

Currently, it's not possible to copy image data between qubes.

Reproducibility:

1. Copy an image to the clipboard in an AppVM (say, by clicking "Copy image" in Signal)
2. Press the global copy shortcut
3. Nothing happens (no notification)
4. Press the global paste shortcut into some AppVM
5. Nothing happens (no notification)
6. Paste inside the AppVM
7. No image is pasted.

The solution you'd like

Allow pasting image data between qubes.

The value to a user and who that user might be

Anyone who wants to copy an image from one qube to another (very frequent).

Completion criteria checklist

No response

[alimirjamali]

@andrewdavidwong I would recommend removing C: core label and adding gui-virtualization label.