Button to open notification settings

[jariz]

A lot of settings people are requesting are perfectly possible through the system preferences already (#47, #64). Maybe we should add a button to Noti's settings that opens this preference pane.

@mirshko: any idea how to do this elegantly from a UX perspective? We'd preferably make clear that the following settings can be changed:

• Muting sounds
• Alert style (stay on screen yes/no)

But I don't want it to have too much copy though.