This makes it possible to id resources by an md5 sum.

It also prepares for deferred resource loading by splitting
the image() method of some resources into the image used
for icons, and the actual resource data used as e.g. brush
or pattern.

Squashed commit of the following:

commit aaf181254f8fe3800c589687566e6036ed100f5d
Author: Boudewijn Rempt <boud@valdyas.org>
Date:   Thu May 1 14:06:57 2014 +0200

    Make saving/loading tags work again

    Tags now work primarily with md5, then with the filename without the
    path. In some places, Krita was already adapted to work with just the
    file names, in others not yet.

    Also: reinstate the KoResourceTaggingTest and make it actually work
    without depending on a given user setting.

commit 4cdc4e1e11a8ae550e1104108d929d4052ba3411
Author: Boudewijn Rempt <boud@valdyas.org>
Date:   Thu May 1 11:31:49 2014 +0200

    Add methods to get a resource by filename or md5

    The filename is stripped o...