Match client layout when using monitor mode

[TijZwa]

Is your feature request related to a problem? Please describe.
#3524 implements monitor mode. This works great. It would be very cool if the server can match the client's geometry, like Windows Terminal Server does.

Describe the solution you'd like
After the clients send it's ["desktop-size"] hello packet; create an XDUMMY display for each client display in a left-to-right order.
Then match the resolution of clients displays.

Describe alternatives you've considered
Using xrandr commands on the server to match the clients geo.

[totaam]

We need a new switch - not sure we should overload the resize command line option on the client to make this request.
Together with --desktop-fullscreen=true, we should do our best to take over the client's desktop area.

[totaam]

There was already some support for this on the client side:

xpra/xpra/client/ui_client_base.py

Lines 551 to 560 in 90424bb

	def _process_new_window(self, packet):
	window = super()._process_new_window(packet)
	if self.desktop_fullscreen and self._remote_server_mode in ("X11 desktop", "X11 monitor"):
	from gi.repository import Gdk # @UnresolvedImport
	screen = Gdk.Screen.get_default()
	n = screen.get_n_monitors()
	monitor = (len(self._id_to_window)-1) % n
	window.fullscreen_on_monitor(screen, monitor)
	log("fullscreen_on_monitor: %i", monitor)
	return window

The commit above triggers now the same server code we already used in seamless mode for mirroring the client's monitor layout.
The key for clients is to specify desktop-fullscreen = True in the hello packet.
To configure the virtual monitors, the server will use the new (v4.4) monitors hello packet attribute or fallback to the legacy screen_sizes one.
@TijZwa can you give it a spin?

Here is an example of monitors definitions:

    monitors = {
        0 : {
            "name"      : "DP-2",
            "primary"   : True,
            "width"     : 3840,
            "height"    : 2160,
            "x"         : 3840,
            "y"         : 0,
            "mm-width"  : 708,
            "mm-height" : 398,
            "automatic" : True,
            },
        1 : {
            "name"      : "DP-0",
            "primary"   : False,
            "width"     : 3840,
            "height"    : 2160,
            "x"         : 0,
            "y"         : 0,
            "mm-width"  : 597,
            "mm-height" : 336,
            "automatic" : True,
            },
        }

[totaam]

@TijZwa I assume that this works for you?