[volumecontrol] Ensure safe volume in long listening time event.
[volumecontrol] No need to show warning when data about high volume step is received.

HighVolume D-Bus signal is used to communicate possible HighVolume step
in the current audio route. SafeLevel 0 means no HighVolume step (all steps are
ok), SafeLevel > 0 means any volume values over SafeLevel are considered
harmful in longer exposure.
[volumecontrol] Be more strict about volume warning dialogs.

Adhere more closely to European Standard EN 60950-1/A12.
Merge pull request #197 from jusa/volume

Adhere more closely to European Standard listening safety instructions.