Fix libpng warning about png files

Fixes #18

Signed-off-by: Guillaume Clement <gclement@baobob.org>
Update french translation

Some strings were moved from song_widget to song_model, update and
validate them.

References #17

Signed-off-by: Guillaume Clement <gclement@baobob.org>
Add back some missing funtionality from before model split.

This is needed to get the same ambount of functionality as before.

This references task #17.

Signed-off-by: Guillaume Clement <gclement@baobob.org>
Speed up tree rendering a bit

Telling the tree that all rows have the same height allows to spee up
the rendering.

Signed-off-by: Guillaume Clement <gclement@baobob.org>
Remove unused header inclusion.

Signed-off-by: Guillaume Clement <gclement@baobob.org>
Update the model when some information about songs change.

This is needed to complete #17.

Signed-off-by: Guillaume Clement <gclement@baobob.org>