Add OperationList to keep track of Operations

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.Operations. 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.