* glom/libglom/connectionpool.cc: add_column(), drop_column(),

	* glom/libglom/connectionpool_backends/backend.[h|cc]:
	* glom/libglom/connectionpool_backends/postgres.cc
	* glom/libglom/connectionpool_backends/sqlite.[h|cc]:
	add_column(), drop_column(): Return a bool to indicate failure.
	change_columns(): Report false return values from these.

	* Makefile_tests.am
	* tests/test_selfhosting_new_then_change_columns.cc: Add a simple
	test, though we do not yet test that the expected change really
	happened.