Which discovers/remove/add/ban/unban/reload folders in a loop

Fix #204

This will allow us to avoid useless tasks by using some common code
during the enqueueing step

Since we will soon start inserting at the front or in the middle of
the task list

Which basically replicates the concept of golang's defer

Use #pragma once for double inclusion check

refs #205

We can't know if a device is known based on its sole UUID, but this
isn't used anymore

refs #205

This simplifies debugging since the missing trigger will be displayed,
instead of an unclear assertion failure

Deleting the associated media is enough

When sharing thumbnails, this might end up copying a thumbnail over
itself, which would first remove the existing destination, but since
source and destination are the same file, this would end up just
removing the source and not copying anything

We now favor album thumbnails for album & media. If an album cover is
available, it will be assigned to the album and all media, even if they
have an embedded cover.
We also propagate embedded covers to all media & the album if they
didn't have embedded artwork or a cover file available

Since the album can't have an artwork if no tracks have one.

Fix #201

Instead of relying on the content of m_fsFactories, just use a boolean
This also fix a few thread unsafe accesses

Instead of doing it during the initialization.
This now also handles network devices, which shouldn't be refreshed from
the main thread (or any other thread that's not a background
medialibrary thread, as the network device refresh may block for a few
seconds)
This reverts 7502a15b

Which will... refresh all devices, but from a background thread

Regardless of a specific fs factory, as we might not have the fs factory
that was present to handle some devices that are still in database

One will fetch the devices to be refreshed, one will handle a specific
device refresh