Simplify EActivity.

With unintrusive error dialogs gone, we can cut some unnecessary bits
out of EActivity.

I'm also adding a new enum property called "state", which is one of:

    E_ACTIVITY_RUNNING
    E_ACTIVITY_WAITING
    E_ACTIVITY_CANCELLED
    E_ACTIVITY_COMPLETED

The state of an activity must be explicitly changed.  In particular,
when the user cancels an activity the state should be set only after
confirming the operation has been cancelled and not when cancellation
is requested (e.g. after receiving a G_IO_ERROR_CANCELLED, not when
the GCancellable emits "cancelled").  EActivityBar and EActivityProxy
widgets have been updated to make this distinction clearer in the UI.

E_ACTIVITY_WAITING will be used when activities have to be queued and
dispatched in sequence, which I haven't written yet.

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