The code in `Contacts.MainWindow` that has dealt with operations is
problematic in several ways:

* It only kept track of the last operation, and didn't do that well
  either as it could easily be overwritten
* Due to `Contacts.DeleteOperation` being irreversible, it needed
  special (and buggy) workarounds
* It repeated code for dealing with operations several times, which lead
  to copy-paste bugs.

This commit tries to fix that by introducing a `Contacts.OperationList`
object, which acts as a container and wrapper API for
`Contacts.Operation`s. The most prominent extra API it provides, is that
of postponing execution with a timeout, and being able to cancel ongoing
operations. Both of these APIs allow us to remove any special-casing we
had to do for delete operations.

It also adds a `flush` API, which we can later use to prevent the
application from completely quitting before all operations have
finished executing.