Fix DND of songs in the current playlist

Several bugs here:

* Wrong usage of the new API from PyGObject compared to pygtk (wrong
  methods usage, wrong attributes, etc.)
* the selection's data is now always a bytes string
* the row to insert into a model must be len()-able, which TreeModelRow
  isn't.

This fixes some stuff for #37. There still a scrolling issue though
(scroll back to some weird place after DND).
Factorized code which moves songs in the current playlist on DND

It just variable aliasing, better names and only one block of code left
which does the actual move instead of code spread and copy/paste all over
the function.
Scroll to the selection after DND songs in the current playlist.
Update Changelog entry with DND issues
Fix multiple issues related to drang-and-drop in the current playlist

Merge branch 'fix-dnd-37'

Fix #37
lyrics: homogenize lyrics search patterns before calling plugins

Plugins don't have to do it by themselves now.
It also fixes a bug in the Redirect detection feature of the lyricwiki
plugin, which was capitalizing search patterns before each requests, even
if the redirect page asked *not to* capitalize.
lyrics: more logging informations