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

To help when a const reference is needed as the return value.

This will be used by all classes using the Thumbnail class

Which was created as is_track_presentAFTER, which apparently defaults to
a BEFORE trigger, causing the wrong is_present value to be used

Comparing for equality doesn't work anymore since we removed a Task.
Simply check that all expected steps have been completed, regardless of
another task being completed as well.

It is now up to the user to request thumbnails generation.
It allows lazy loading, and prevents us from potentially generating
thousands of thumbnails for nothing when indexing an entire network
drive.

We don't need to know if a video track was detected or not anymore