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Fix #35

All request that could return more than one row (ie. all member function
that used to return an std::vector<std::shared_ptr<T>> will now return a std::unique_ptr<Query<T>>
The query object can return:
- The amount of row represented by that query (->count())
- A subset of the rows (->items(count, offset))
- All rows (->all())

Instead of doing it after fetching. This will ease up pagination

This was working so far as we were using the cached version. Now that we
are doing the filtering through sqlite, the cached version doesn't help

So its type matches with what sqlite_bind_* functions expect

when fetching lists of albums. This will ease up sharing sorting code

So we can use File.is_present and to simplify integration of Sorting
for search results

This has the extra benefits of reducing our use of Media.is_present

Fix #34

To be able to specify the foreign key relationship on all subsequently
created tables

Instead of potentially using a thumbnail from an album the artist is
only featuring on

Since it will soon also allow to change the Origin field