Resources: Remove KoResourceLoaderThread

All resources were loaded synchronously by now, so KoResourceLoaderThread
was obsolete. This also moved the bundle server to it's own singleton,
since it needs a completely instantiated singleton of KisResourceServerProvider.

KisApplication now instantiates the singletons in the correct order, without
superfluously getting the actual resource providers. Since the order is now
properly known, we can clear the empty krita 3 system preset tags from the
tag database after loading the last bundles. However, the tag system seems
to be case-insensitive, but we used to have both ink and Ink; that still
needs fixing.

This fixes item 2 from

CCBUG:391576

Curiously enough, this also makes Krita start quite a bit faster for me...