Split mif warnings

We now test whether delete or chgrp will attempt to split a Fileset BEFORE we go ahead.

To test:
 - Try to chgrp part of a MIF (via selecting Project(s) or Dataset(s) or Image(s)
 - The "choose target" dialog is unchanged, but on "OK" you should see a warning that this will split a MIF and the option to "Move All", which should move all the data.
 - Bonus: Choose multiple Datasets with multiple split MIFs. Do all the MIF images end up in the appropriate Dataset?
 - Repeat a couple of times, Cancel sometimes etc to check that the dialog is re-used correctly each time.

 - Repeat above with delete. You will only see a "Cancel" option.

![screen shot 2013-05-28 at 22 10 30](https://f.cloud.github.com/assets/900055/575343/4ed81c58-c7db-11e2-9cc3-796799e064a0.png)
![screen shot 2013-05-28 at 22 12 03](https://f.cloud.github.com/assets/900055/575344/51fbef4a-c7db-11e2-856a-b0af542c6733.png)