Namespace hierarchies

[Bart Cerneels]

...continuing a discussion started on IRC.

In core we have several new namespaces: Meta, Collections, Podcasts,
Playlists, ...

There are 2 options for namespace hierarchies in the implementations:

1) grouped by implementation backend:
ex. for Sql
Sql::Collecion
Sql::Meta::Track
Sql::Playlist
Sql::Podcast::Episode

2) or by core namespace:
Collections::SqlCollection
Meta::Sql::Track
Playlists::Sql::Playlist
Podcasts::Sql::Episode

Before any more big commits are made we should discuss.

Note that we already use both in a way:
- Podcasts::SqlPodcastChannel though it's generally agreed that this
is misuse of the namespace.
- MetaFile::Track

I think 1) is superior because it saves massively on typing (ex):

using namespace Sql:::Podcast;

QString
Channel::title()
{}

Bart