Update supported platforms and feature in QtBluetooth

The Low Energy API is final from Qt 5.5 onwards and OSX and iOS are
platform additions.

Change-Id: I73f18b3169c5e1438af4d90c6655eabb515e0d39
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>