wpa_gui: Add tray icon based signal strength meter

wpa_gui: Add tray icon based signal strength meter

System tray icon can be set to 5 different pictographs according to the
connection status. One for disconnected state (not associated with the
network, or not connected with the wpa_supplicant service), and four for
connected status (showing the signal strength on the receiver).

By default this functionality is disabled. The reason for this, is the
fact, that the underlaying approach of this functionality is poll based,
which might be considered as a non-efficient one. Update interval has to
be set explicitly by the user with '-m<seconds>' command line argument.

Status icon names are based on various Gnome icon packs (e.g., Faba).
When icon can not be found, default one is shown (wpa_gui logo).

Signed-off-by: Arkadiusz Bokowy <arkadiusz.bokowy@gmail.com>