Moved some rules around for this - namely, the 'cursor: move' rules for dragging stuff.

Make sure that the blocktypes are draggable when you switch tabs.

There's an obscure bug still remaining where sometimes the blocktype isn't added where it should go. Firefox is not respecting the z-index of the dragged elements also.

There's a couple of small things left to go:

 * Need to have a zone outside of which when the blocktype is dropped, the action is canceled
 * When switching tabs, the blocktypes on the new tab should be made draggable

When handling a request for a new blocktype, return the rendered blocktype if we are doing the request via AJAX, so it can be inserted immediately.

Moved the rendering of the block instance container to BlockInstance::render to facilitate this change.

Added a language string for confirming deleting a blockinstance. Also changed other language strings to refer to them as 'blocks', users probably would get confused by the 'instance' part.

Now the user is asked for confirmation before a blockinstance is deleted. To be implemented in the static version later.

Also, removed a bunch of redundant error cases.

The performance fixes largely centre around not responding to _every_ mouse event.

The ajax request is accompanied by some code to find out where the block is being moved to.

more tidyups - view_process_changes now detects defined(JSON) rather than accepting $ajax as a parameter, added some more documentation

made the removeblocktype view function called blockinstance->delete rather than access the database itself

added support for ajax adding and removing columns. I changed view_process_changes so it works from either a normal post or an ajax request, view class methods now take a 'returndata' argument for required ajax return data. The javascript rewriting buttons has changed to match what the normal buttons are called so that view_process_changes can read them

sends data back to database, reads data from database.

Getting the reordering done with the unique constraint on view/column/order
was .. interesting ...

Next: ajax version

changed the placeholder block type to just return a string rather than futsing with the configdata stuff

fixed up some stuff in dml around quoting of identifiers and added prepared statements to execute_sql

Basically, it all works now except for doing the actual AJAX request (10 minute job) and adding blockinstances to an empty column. The hotzones at the bottom of columns have been extended to the end of the content, for ease of use.

Without the outlines, and with a 'snap' of five, it doesn't perform too badly, but there is probably lots of improvement to be had still. Some kind of data structure storing the layout of columns might be useful, but also may cause problems if it gets out of sync.