xorg: list the server's windows, and cut a window's screenshot from the screen (hq issue 344)

A reminder window that showed as transparent and empty could not be looked at: the window manager's
list leaves dialogs out and says nothing a compositor goes by, and xwd -id refused a window of a
32-bit visual with BadColor (X_QueryColors). xorg_windows answers every window with its class, and
on request its depth, type, state, role and own opacity; a window's screenshot is now cut from the
whole screen as the compositor shows it, and inline answers the image for a session that cannot
read the machine's files. Declares xorg-xwininfo, which both now use.

modules/xorg/module.json

@@ -29,6 +29,7 @@
     "xorg_input_set",
     "xorg_keyboard",
     "xorg_screenshot",
+    "xorg_windows",
     "xorg_x_log"
   ],
   "resources": [
@@ -72,6 +73,11 @@
       "type": "package",
       "package": "xorg-xwd"
     },
+    {
+      "id": "package-xorg-xwininfo",
+      "type": "package",
+      "package": "xorg-xwininfo"
+    },
     {
       "id": "package-autorandr",
       "type": "package",