Allow copy-paste to other programs

[ecations]

Hi!

At the moment copy-pasting doesn't seem to work between Pixelorama and other applications. It would be really useful to be able to copy paste artwork to other software, for example when building larger tilesets.

[OverloadedOrama]

It may be possible once godotengine/godot#63826 is merged in Godot, although support for clipboard set image also needs to be added. We might need to update to Godot 4.x first in order to implement this though, because I don't know how likely it is for this feature to get backported to Godot 3.x.