Unfortunately, the implementation was not thread safe and was not
sticking to the thread local main context.
In addition to this, it is not entirely clear to me how this API should
behave. The current approach is to simply cancel the transition with the
state machine halting in its current state. Instead, it could also make
sense for cancellation to cause the state machine to return a
G_IO_ERROR_CANCELLED.
As such, simply remove the feature for now. If anyone actually has a
good use-case then we can add it again.
In some situations one may want to guarantee that the last steps of an
SSM are run even when the SSM is completed early or failed.
This can easily be done by making fpi_ssm_mark_completed jump to the
next cleanup stage when called (this also includes mark_failed). Due to
the mechanism, it is still possible to explicitly jump cleanup states by
using fpi_ssm_jump_to_state, including a jump to the final state in
order to skip all cleanup states.
