report progress for triggered actions

APT doesn't know which packages will be triggered in the course of
actions, so it can't plan to see them for progress beforehand, but if it
sees that dpkg says that a package was triggered we can add additional
states. This is pretty much magic – after all it sets back the progress
– and there are cornercases in which this will result in incorrect
totals (package in partial states may or may not loose trigger states),
but the worst which can happen is that the progress is slightly
incorrect and doesn't reach 100%, but so be it. Better than being stuck
at 100% for a while as apt isn't realizing that a bunch of triggers
still need to be processed.