[REFACTOR] Removes unnecessary core.monitoring classes

Removed IProgressMonitor and ISubMonitor, started to use SubProgressMonitor. Cleaned up ui.widgets.progress.

Change-Id: I8d3a09663a04dc8936fca3815cab0d5f77d2dc3a
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1861
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Holger Schmeisky <google@schmeisky.com>