desktop menu entry for starting shadow server (xdg)

[Antoine Martin]

MS Windows already has one (and macos can't do menu entries at all, only ".app" packaging).
Made a little easier by automatic port allocation (#1945).

[Antoine Martin]

Added in r20291 + r20292 (tiny change - mostly packaging).

There is now a "Xpra Shadow Server" menu entry in the system application menu.
Running it will start a shadow server which can be controlled through its system tray menu icon (add ​TopIcons-plus if you're on gnome3 without a working system tray by default) or via the usual xpra control, xpra stop, etc.
This server also shows up via the mdns session browser.

It listens on a random unprivileged TCP port (thanks to #1945). Though those ports are usually blocked by the firewall, so maybe we shouldn't bother? (then people don't try and fail to connect to it)
r20292 moves the priority of ssh connections in the mdns gui (since we have better ssh integration, including prompts for passwords and passphrases: #1646), so those will be preferred to tcp sockets.
We don't enable SSL on those TCP sockets because the system-wide cert we create during installation is only accessible by root. Creating a cert for each user would be possible, but probably not wise.

@maxmylyn: FYI, feel free to just close.

[J. Max Mena]

Noted and closing.