Introduce Pairable property.

Introduce Pairable property.

As the name suggest it tells when an adapter enter or leaves bondable
mode, it works as follow:
Pairable=true & Discoverable=true: limited IAC
Pairable=true & Discoverable=false: pairable but inquiry scan disabled
Pairable=false & Discoverable=true: inquiry scan enabled (with Generic IAC)
but new pairings cannot be created (same as if there was no agent)
Pairable=false & Discoverable=false: inquiry scan disabled & non-pairable
(as if there was no agent)