[Muine] Multimedia Key Support

[Lee Willis <lee leewillis co uk>]

The patch below adds basic support for the following multimedia keys:

- Pause (Acts as a Play/Pause toggle as per the Muine toolbar)
- Next
- Previous

There are a few rough edges to be tidied up, namely:

(1) Once in MmKeys.cs we should probably emit signals rather than
calling playlist.* directly
(2) We need to carry out the playlist checks, but we can't really from
where we are without making lots of stuff unnecessarily public. This
will probably be solved if we fix (1)
- It's only been tested on my local machine - all feedback welcome!

http://www.leewillis.co.uk/muine/mmkeys.patch

Lee