Add raise / lower volume buttons as in the web version

[Fabrizio Benedetti]

The web version on myNoise.net features two buttons, in the group of buttons under the sliders, that raise and lower (respectively) the level of all the sliders. These would also be very useful in the app.

[V]

I agree - many of the presets I like on web are useless in the app because they are too loud or quiet compared to other apps.