Add a GCancellable to EActivity.

EActivity now uses a GCancellable to manage cancellations, instead of
having its own redundant cancellation API.  API changes are as follows:

  + e_activity_get_cancellable()
  + e_activity_set_cancellable()

  - e_activity_cancel()
  - e_activity_is_cancelled()
  - e_activity_get_allow_cancel()
  - e_activity_set_allow_cancel()

EActivity's "cancelled" signal remains, but only as a repeater for
GCancellable::cancelled signals.  It should not be emitted directly.

The presence of a GCancellable implies that cancellation is allowed.
EActivity does not create its own default GCancellable, it has to be
given one.

If a CamelOperation (cast as a GCancellable) is given, EActivity will
configure itself to listen for status updates from the CamelOperation
and propagate the information to its own "primary-text" and "percent"
properties.

These changes allowed me to start cleaning up some of the incredibly
convoluted logic in mail-mt.c -- in particular, mail_operation_status()
is completely gone now.  mail-mt.c is still in a transitional state --
much more significant changes coming soon.

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