Fixing sound applet issues #2807 and #3284

#2807: There was an issue that when sliding:

1. 1% with "low" icon
2. 0% with "low" icon
3. 0% with "mute" icon

In https://github.com/pixunil/Cinnamon/commit/e8a22a4046032aef26caba5f1db67b880037bdda step 2 is removed with a better mute handling [line 1133 - 1144]

A not reported issue was also closed: when the slider is at 100% and you scroll to decrement the value by 5% the value was 94% then 89% ..., now the expected value 95%, 90% ...

#3284: Added just two event handlers for the event `scroll-event`
https://github.com/pixunil/Cinnamon/commit/5f0712a50254450f8ccf306e9d4d4d123dedc77a
If the mute state is updated first, the applet does not know the current volume, so I put the update mute procedure _after_ the update volume call.