[Feature Request/Bug] Add the settings for notification sounds

[mankysee]

Firstly: It's a great program and I thank you for it!

I'm using depecher 0.6.1-2 and I have a problem with notification sounds: They are too loud and there is no way to tone them down. You can lower their volume by lowering the volume of the current ambience but it's a cumbersome way since I don't want to lower the ringtone volume for example I want be able to lower or mute notifications' sounds of depecher alone.

So as a way to address this problem I propose additional app settings: Notifications' sounds volume(using as maximum volume a volume level set by ambience) and Mute Notifications.
This way a user can customize this aspect any way she wants.

On a side note in the sailorgram there was such setting(Mute to be precise) actually and it had it's own button on the app cover allowing to enable and disable notification sounds from the sailfish home screen.