Added QML API for getting the limits for a specific radio band.

Added QML API for getting the limits for a specific radio band.

Using properties for the frequency step, minimum and maximum
frequency of the currently selected band.
Also updated the declarative-radio example to use the minimum
and maximum frequencies to show a tuner band for the radio.

Change-Id: I9f28f10e98e008c14b10bdc12b0727086cd45f0b
Reviewed-on: http://codereview.qt.nokia.com/3829
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>