Support for unremovable groups

Hi,
there is an issue in the collect repo: https://github.com/opendatakit/collect/issues/185. I would like to add support for unremovable groups to avoid deleting them.

I've already prepared a solution but I should have created a thread here before.

So my idea is to use additional attribute: `removable="false()"`it would be something like:
``
thanks to that I'm able to implement all required changes quite easy: https://github.com/opendatakit/collect/pull/1190/files

What do you think?
Here is an example form:
[group.txt](https://github.com/opendatakit/xforms-spec/files/1097729/group.txt)

Start with the linked collect issue 185 and the proposed collect pull request 1190, then inspect the example form in group.txt. Determine how the removable="false()" attribute should be represented in the XForms specification and what behavior the example is intended to require; done means the specification decision and expected behavior are unambiguous.

Written by the indexing model from the issue text.