Issue #86: DoorhangerPrompt: Builder for creating a prompt Doorhanger providing a way to present decisions to users.

This reverts commit 899fa2d7.

Optional for filename to be configurable

- Implement test API functionality
- Make library tests more concise by using test API features
- Add tests for test API throwing NPE

At this point in the stack, we're not in control over what our
autocomplete providers are, what actions they'll do in order to
field our queries, etc. For example, some providers may hit the disk
and perform expensive DB queries internally. Some may even hit the
network, in theory!

In order to keep things perceptively speedy, let's run the actual work
off the main thread. This patch sets up a new pool thread to process
autocomplete requests. More than one thread is selected so that we maintain
liveliness during quick user input. Background tasks are cancelled as new
queries come in, and stale results are discarded.