Issue 75: Multiple song support in OSC server

- Modipulate-OSC now keeps track of multiple songs (closes #75)
- No longer possible to make a song unreachable: reusing a song ID unloads previous occupant
- OSC address patterns which are song-specific updated to take song ID int argument
- OSC address patterns renamed for some consistency (#76)
- Pure Data patch updated to support the above

Checkout branch issue-75 and taste the :rainbow: