Notification sound option picker should select a sane default

[gergles]

I imagine most people will want to use one of the system notification sounds for their chat notifications, so perhaps the file picker for those should start in /System/Library/Sounds instead of ~/Documents.

Also, adding a preference to have a default notification sound for any chats that don't have one specified might be nice.

[avaidyam]

Good point - I'll design a quick control to mimic how Messages does it. /System/Library/PrivateFrameworks/ToneLibrary.framework/Resources also has all of the "modern" contact sounds heard on iOS and in Messages on macOS. I've thought about what to do with that; should we copy them over at runtime or copy them over at build time (which increases the binary size by a fair amount - 10MB), or just ignore them?