First time install: multiple platforms, fix shell disposed.

First time install: multiple platforms, fix shell disposed.

Fixes for the first time install support:

- Support installing more than one platform.
  The custom install task was tailored before to one
  install a single package and return one status with
  on archive install path. This has been made more
  generic.

- Fix a couple occurences of the dreaded "SWT Shell disposed"
  error that happen when updating the progress bar and closing
  the window at the same time. The issue is that I was checking
  isDisposed before starting a syncExec but the window can be
  closed between the moment the syncExec is schedule and when it
  happens so code must check again for isDisposed inside the
  syncExec.

Change-Id: I413cfacd63406febc7953effad5ece2ef2854360