Due to the reliance of a one-time job's signal delivery, the
Dependencies button went missing after the second time resource's page
is opened. This patch refactor the whole task into:

- a job class which collects a list of resources from PackageKit
  transaction;
- a job runner class which launches the job on demand, keeps the result,
  and takes care not to launch two jobs simultaneously.
- a data class (QGadget) to replace JSON object approach.

The runner is lazy, and also supports changing package ID (in case it
changes after install/remove operation).

As a drive-by change, refactor list view delegate to wrap the whole
summary text, because there is absolutely no reason for eliding.