Refactored the location of the progress_callback so that the UI can
specify it and ensure it is used by the automagics process as well as
the plugins themselves.
The progress_callback function has also had an optional description parameter
added, so that it can differentiated in case two scans happen
simultaneously.
We shouldn't be stacking unless we're required, so now
we run after the construction phase, and run our own construction
phase is we've changed anything.
This change is quite signficant, and requires that TranslationLayers
get all additional parameters that they need through their requirements.
These are now automatically enumerated and populated on object
construction based on the requirements, so should not require lots of
repetitive filling out of fields.
It does come with the downside that TranslationLayers can only be
contructed with a context (and appropiate config), but TLs in particular
always require a context (to contain the base layer) and blank configs
can be constructed relatively easily (convenience functions can be added
if necessary).
This allows configuration trees to be built up, and their configs
spliced into an existing config (as if it were being loaded from a
file).
Not all ConstructableRequirements use this method, since SymbolTables
(for example) do not have access to the context or config_path in order
to get to any parameters stored in the context's config. They therefore
are still passed their requirement values as __init__ parameters
instead.
Mike Auty