Debug image unloading on invalid files

A bit tricky to unload various form references in the proper order - all
should now be resolved.
Do not use "Form.Caption = <string>" to change image window captions

VB must use more than just SetWindowText when changing captions, because
changing a form's .Caption property causes docked windows to un-dock.

pdWindowManager now includes a dedicated function for requesting image
window captions, and it will not un-dock docked windows.
Update FreeImage image load code style

Minor changes only
User-facing options for show/hide individual toolboxes

(Relates to #93)

This feature had more interactions than I anticipated, but I *think*
I've worked out all the kinks.

The biggest problem is that toolbar forms must never be unloaded.
Whether docked or floating, we need to keep toolbar forms in memory so
we can properly track and sync their position - otherwise, if/when the
user re-activates a toolbar, it won't appear in the proper place.
(Also, things like Undo/Redo button en/disabling needs to be applied,
even if a toolbar is not visible, so that if it is *reactivated*, all
buttons are in the proper state.)

To that end, toolbar windows are never unloaded, only hidden.  This
should be properly handled, even for things like clicking the control
box X on a toolbar form, and the Window menu items should be
automatically synced with any visibility changes.

One thing I very much like about this change is that it allows for a
truly minimal PD interface!