This allows to have an automatic cleanup of the timeout source when the
the callback is reached and to avoid to do further state changes in the
middle.
Allow to pass a double-pointer to be nullified as the transfer data in order
to mark it as NULL when the transfer is done.
This is useful if we're keeping the transfer around in order to check that
no one is currently running.
Use the same approach of GTask, making possible to set the data from a
function. Givent the fact that a SSM has now a device parameter, it's
generally not needed to pass an extra data value.
In such case make it possible to set it and to define a destroy-notify
function to handle its destruction when freeing the SSM.
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.
Pass the struct fp_dev and user_data to fpi_ssm callbacks, so that we
might be able to get rid of the fpi_ssm_get_user_data(), and
fpi_ssm_get_dev() as most drivers just get those from the ssm anyway
in their callbacks.
