gom: Remove fetching the generated SQL for a command

We ultimately want to get rid of SQL in the public API, so start
with not making the generated SQL command readable.
gom: Remove incorrect warning in gom_resource_save_sync()

The API documentation for gom_resource_save_sync() mentioned that
this needed to be called within the SQLite thread. This is incorrect
since b9173234ffc848083724486dd564af4a9ee89884 which made this
sync call push the request to the SQLite in a thread-safe way.
tests: Add test for column added in DB version 2
gom: Don't consider a property mapped if type is not handled

If the type for a property is not one of the fundamental types that
we know how to handle, don't map it in the command builder.
gom: Consider version 0 the absence of version

If version 0 is the absence of DB version information in the database,
then migrations should be done at least up to version 1.
gom: Add GomResource property metadata

Add "new-in-version" and "is-mapped" metadata for GomResource
sub-class properties. This allows application developers to
control which properties will be mapped to column names, and
tell the automated migration tool which columns to add.
gom: Add gom_command_builder_build_create()

As documented in the API docs. This will be used for the automatic
migration feature.
gom: Add gom_repository_automatic_migrate_(a)sync

It's an automatic migration tool, which will take care of create the
table for each of the passed types, as well as adding any columns
necessary to upgrade to the latest version.
tests: Use automatic migration in test