Allow downloads to be filtered by draft status

Generally new features are only being included in the 7.x-4.x branch. Webform 3.x is maintenance-only at this point. However since you included backports, we could probably make this happen for previous versions.

Unfortunately there were a number of issues that need to be addressed because of changes in the 4.x branch:

- This patch does not work on "latest x submissions" feature, since it will filter the list of submissions AFTER the list of latest SIDs has been retrieved.
- This patch doesn't work after the introduction of #1327186: Use BatchAPI to Export very large data sets to CSV/Excel (admittedly that happened at the same time as your patches)
- UI-wise, I think it would be better to put this set of radio buttons under the "Download range" options. After the changes to the way downloading works, it actually MUST be located inside this fieldset in order for the settings to make it to the all the necessary functions.

I rerolled the patch to work with 7.x-4.x and updated the naming conventions to match other range options (e.g. calling it "completion_type" to match "range_type" instead of "draft_options").

If you'd like this patch backported to 7.x-3.x and 6.x-3.x, I can accomodate that if you reroll the patch based on this changed version. In the mean time, I've committed this version to the 7.x-4.x version.

7.x-3.x is receiving critical bug fixes only.