Pga3 Query Builder

As some of you may have noticed, I've been tinkering with the Query
Builder code over the last few days. Currently I've done the following:

- Updated to use ctlSQLResult, and batch loading code as per the Query
Tool.
- Added messages and history text boxes.
- Fixed explain.
- Allow removal of joins.
- Fix for proper use with schemas
- Made menus and toolbar match Query Tool as appropriate.

Current known problems are:

- Double clicking columns opens the join dialogue. This appears to be
because wxWindows detects the double click as a drag-drop, which then
overrides the double click event. I'm not sure how to fix this - think
it needs Andreas' talents to fix :-)

- It is currently possible to try to set criteria for * columns. I've
not tried it, but it will probably fail spectacularly!

Anyway, I would appreciate any feedback...

Cheers, Dave.