BUG Fixed issue in file upload via the HTML Editor media dialogue

Fixed issue where file upload via the HTML Editor media dialogue would not prompt users to overwrite existing files.

This resolves https://github.com/silverstripe/silverstripe-framework/issues/2079 where the upload field would silently ignore already existing files when `UploadField::setOverwriteWarning` was set to true.

This resolves this issue by displaying all in progress uploads and allows user interaction with failed uploads (such as when an existing file is on the filesystem).

The screenshot demonstrates the two different queues: The progress queue has been styled grey for distinction.

![image](https://f.cloud.github.com/assets/936064/679141/05ef8f46-d959-11e2-81b0-cf81ed18522a.png)

This form could still be enhanced (such as providing other options for failed downloads), but it's sufficient to solve the outstanding issue in the simplest way possible.