Fix issues when importing the same resource several times

This patch should fix two bugs: IS-446588,IS-446743

Please review the patchset in a per-patch basis, it is easier.

Note about bug IS-446588: Krita still asks whether it should overwrite the resource with the same filename, but when the user agrees Krita skips removing the resource (and all its history, if exists). Instead the import code just returns the existing resource.

I'm not sure it is the expected behavior, it is rather disputable. At least Krita now correctly detects the duplicated resource's MD5 and skips actual overwriting.