This adds a number of new internal states to better capture what is
going on. Also added are checks that all transitions we make are in the
set of expected and valid transitions.
Only three drivers use the state_change notification. These drivers are
updated accordingly.
Image devices are simply deactivated suddenly. As such, the cancellation
logic of FpDevice is not really useful there, but the documentation was
apparently accidentally copied unmodified.
This is a rewrite of the core based on GObject and Gio. This commit
breaks the build in a lot of ways, but basic functionality will start
working again with the next commits.
