send more than one clipboard target to greedy clients

[totaam]

This is needed to be able to provide both text/html and text/plain targets to the html5 client: Xpra-org/xpra-html5#283

At the moment, we just choose the first match from the preferred order list:

xpra/xpra/x11/gtk/clipboard.py

Lines 436 to 442 in 9db2102

	# find the preferred targets:
	targets = self.choose_targets(otargets)
	log(f"choose_targets({otargets})={targets}")
	if not targets:
	send_token_with_targets()
	return
	target = targets[0]

The most difficult part is to make the data fit in the clipboard-token packet, which has been overloaded too many times already.
The correct way to handle this is to use the new packet format: #1942 so that we can just send a dictionary of targets + values.