More code completion fixes

More code completion fixes

This changeset fixes additional corner cases for XML code completion.

It handles inserting "new" attributes in more places, meaning for
example that if you code complete here <Button ^/> you will still get
"new" completion where the inserted attribute contains ="^".

Similarly the completer also handles inserting full element tags (with
opening and closing tags) in more scenarios, and still avoids doing
this in the case where you are on an opening line of an element which
already contains attributes on subsequent lines.

The new unit tests reveal additional minor corner cases.

Change-Id: I3f1b903e1c1768e4bdd42a5486ce729e348c4da5