WPS support in NM (AP)

When wpa_supplicant is an AP it only supports basic functionality where it is the registrar too.

When connection sharing (as an AP) is active with WPS support, there could simply be a dialog "enroll new device", like this:

+-----------------------------------------------------+
|  Enroll new device                                  |
+-----------------------------------------------------+
| Depending on your device's capabilities, select     |
| an enrollment method.                               |
|                                                     |
| If your device is capable of using PIN enrollment,  |
| have it generate a PIN and enter it here:           |
| PIN from device: [________]  [enroll with PIN]      |
|                                                     |
| Otherwise, it can support the push-button method,   |
| push the button here and on the device to enrol it. |
|                        [enroll with button method]  |
|                                                     |
|                                            [cancel] |
+-----------------------------------------------------+