[mainvolume] Add getter for HighVolumeStep to D-Bus API.

Major revision changes to 2.
Path: /com/meego/mainvolume2
Interface: com.Meego.MainVolume2

New property "HighVolumeStep", unsigned integer variable telling if
current steps contain high volume step. 0 means no high volume step,
value > 0 means high volume step. Safe volume step is high
volume step - 1.

Getting all properties has now also HighVolumeStep appended to values.