file-transfers and printing: read files as needed

[totaam]

Even reading from stdin is handled as one big blob:

xpra/xpra/client/base/command.py

Lines 586 to 598 in 1bff4bc

	if self.filename == "-":
	# replace with filename proposed
	self.filename = command[2]
	# read file from stdin
	with open(sys.stdin.fileno(), mode="rb", closefd=False) as stdin_binary:
	self.file_data = stdin_binary.read()
	log("read %i bytes from stdin", len(self.file_data))
	else:
	if not self.validate_file_size(os.path.getsize(self.filename)):
	return
	from xpra.util.io import load_binary_file
	self.file_data = load_binary_file(self.filename)
	log("read %i bytes from %s", len(self.file_data), self.filename)

Instead, we should be using Gio.InputStream instead.

See also #2990, #3329