Thumbnailing threads now mastered by QThreadPool.
Added configurable presets for content providers.
Fixed typo in comment.
Thumbnailing is now in separate thread pool.
Added new mechanism for safe clearing ThumbnailManager tasks.
Removed useless variable.
Now thumbnail DB interface and workers pool is mastered by Canvas class.
All tag DB interfaces is now have unique names.
TotalTagInterfaceCacheUpdater now controlled by QThreadPool.
ThumbnailDBInterface is now controlled by Canvas.
Main QThreadPool controlled by TagBrowser itself.
ContentProvider destruction is now asynchronous.
Thumbnail database is updated by 50 thumbnails per time.
AsyncRequester destruction will be waited forever until done.