Made BindingList not raise KeyError on remove()

A little lazy programming as it supresses possible misuses, but can
possibly be more robust. Maybe it should warn instead?
Added close all action + docs

Close all simply closes all signals.
Cleared up in progressbar and thread types

Moved UIProgressBar (the one that hooks hyperspy's progressbar) to its
own module, and gave the responsibility of connecting to the importer
(mainwindowlayer2).

Also refactored threaded.py into one Threaded class (old behavior), and
made a ProgressThreaded class for threads with a Modal progressbar. That
one still needs QA, as I can't currently find a decent use case.
Docs and bugfix