Address Brecht's review, mainly:

* Simplifies handling of 'background fallback thread' in scheduler
* Add note (and debug check) that background pools do not get created recursively (from other task pools).

Cleanup: factorize task data freeing into own func, much less verbose that way.

On second thought, add a new API func to create a 'background' task pool.

Will make the patch less verbose since it avoids changing existing code.

Merge branch 'master' into new-filebrowser-preview-temp

Merge branch 'master' into new-filebrowser-preview-temp

Merge branch 'master' into new-filebrowser-preview-temp

BLI_task: add optional 'forced background' behavior to pools, and freefunc to tasks.

Currently, task scheduler in mono-threaded context do not have any worker thread,
which means you have to 'work_and_wait' on a pool to get its tasks done in this context.
This is not suitable for that are to be done in a complete asynchronous, background
fashion, so this commit adds:
* One worker thread to scheduler in case none is created by default;
* That worker thread only execute tasks from pools which have relevant option set
(so-called 'forced background' pools).

This commit also adds an optional freefunc callback to tasks, in case we need
more sofisticated behavior that a mere MEM_freeN...

FileBrowser Preview: Rework code to make proper usage of tasks, pool and scheduler.

So now each and every preview is a task, this allows us to get rid of quite a bit of
stuff. We still need a 'done' queue though (otherwise we'd need to directly update
cached entries from thread/task, which is absolutely not desired!).