A couple of checks to make sure that having blocktype code running when blocktypes aren't installed doesn't kill the admin section completely.

- Fixed a bug in the view_layout install.xml definition
- Added view layouts to core_data

Fixed up some of the admin/extensions/plugins.php stuff to do with
artefacts & blocktypes and their interdependencies.
(there's still a todo in there but it's cosmetic).

moved all the STATEMENTs out of install.xml into postinst, split postinst into two - right after core data & right at the end

Now, table names (and other identifiers that could be confused with SQL kewords), should be put into SQL like this:

SELECT * FROM {artefact}

The braces are matched and expanded by DML now to include the prefix and be properly quoted, which means that tables like 'view' and 'group' don't need renaming (and nor should they, there's little reason why we should rename tables to get around such constraints).

This has removed a whole bunch of $prefix = ... and get_config('dbprefix') stuff, which makes things a little simpler yet again.

Tell the user they cannot use the networking support if they have not installed the appropriate PHP modules

code