wpa_gui-qt4: Add a new window for showing peer information

wpa_gui-qt4: Add a new window for showing peer information

This provides some initial functionality for showing peer information,
i.e., showing information about other devices that has been discovered.
Currently, information is only available in the AP mode (list of
associated stations), but this is expected to increase in the future
(e.g., show the current AP in station mode, other stations in IBSS,
etc.). Furthermore, there will be actions available for doing things
like providing a WPS PIN for a station.