Allow default permissions in hook_imagecache_default_presets() mechanism.

Issue Summary

I really like the simple and robust method of inserting imagecache presets in mymodule_imagecache_default_presets().

However, the preset comes up without any permissions. I want to be able to define minimum permissions in mymodule_imagecache_default_presets() to simplify users life, e.g. allow view by authenticated role and potentially by anonymous.

Then I expect it to be overridable (off or on) on the permissions page, same way as the preset can be overriden. I think "defaults" for permissions are not necessary, but would be nice to have.

For real use case see #575808: Create image preset using hook_imagecache_default_presets.