Add test_selfhosting_new_from_example_operator.

* glom/base_db.[h|cc]: Move add_user() to
* glom/libglom/db_utils.[h|cc]:
* glom/frame_glom.cc:
* glom/mode_design/users/dialog_users_list.cc: Adapted.

* glom/libglom/connectionpool.cc: invalidate_connection():
Also clear m_pFieldTypes to make sure that we refill it when making
a new connection.

* glom/libglom/data_structure/fieldtypes.[h|cc]: Add get_types_count().
* tests/test_selfhosting_utils.[h|cc]: Add test_selfhost() to
start an already-existing .glom system.
* Makefile_tests.am:
* tests/test_selfhosting_new_from_example_operator.cc:
Add this new test which attempts to re-open a .glom system as
a non-developer user. This currently fails to get the list of
tables from the database. I must investigate that.