Implemented maximum file size limit for import

Fix/Workaround for https://trac.openmicroscopy.org.uk/ome/ticket/12012

Importing via Insight is now limited to 2000 files at once; if this limit is exceeded the following error dialog is presented:

![image](https://f.cloud.github.com/assets/6575139/2523876/aca1f72a-b4d7-11e3-84d0-3f7c254b244f.png)

Any suggestions for a better error message text?

To test:
Try to import more than 2000 files; you shouldn't be able to do that.