Playlist save/export options

An alternative solution to #4480 (the other being #4481), and also fixes #1266.  Creates a dialog that shows before saving a playlist to allow the user to set certain export options.  Options are saved for the next time the dialog is used.  The options implemented are:

* File paths: Automatic (existing behaviour, relative if within playlist directory), always relative, or always absolute.
* Path separators: Automatic (use native), Windows-stye (``/``) or Unix-style (``\``).

The default path separator behaviour (use native) represents a change from current behaviour, which is to always use Clementine's internal Unix-style representation.  This can be changed if the new behaviour might be problematic.

![image](https://cloud.githubusercontent.com/assets/12193/3913751/5267ceec-233b-11e4-87f8-c4b76e3d4dda.png)
Usually, we follow Google's style (http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml)
I don't think it explicitly ask for leading underscore, but it is the convention we have (almost) everywhere else for class member.
But anyway, this was just a small remark about consistency (and if it is already like somewhere else...)