Tidying/Simplifying AssistantComposerForm code

and discovering a few bugs.
Clarifying some concepts, for future understanding.
AssistantComposerForm callbacks reorganised.

No known bugs. Everything is working.
Continuing to reorganize and simplify the code
for future reference...
AssistantComposerForm reorganisation complete

Everything is working. I'm going to try replacing
the callbacks class by the corresponding interface.
AssistantComposerForm UI complete.

Removed ComposerFormCallbacks.
Now using the new IPaletteFormsHostForm
interace implemented by the
AssistantComposerForm.
No known bugs. If there are any, they should
be fairly easy to track down.