Context: Add more uniqueness to temp directory names.

- If a single thread attempted to establish multiple package kit
  contexts, it would fail due to a collision between their respective
  temporary directories. As such, use both the thread ID and
  system_time() as a random elements in the directory name to ensure
  this doesn't occur.
HaikuDepot: Stub out various support classes for package kit use.

- DecisionProvider and JobStateListener will be needed in order to
  interact with the package kit in various ways eventually, though
  the implementations are currently all empty.

- Adjust HaikuDepot's PackageManager class to inherit from
  BPackageManager, so as to be able to actually interface with
  the package repositories.
MainWindow: Initial hooks into package kit.

- MainWindow now attempts to refresh all available repositories and
  fetch their respective package lists on startup. Much still remains
  to be done, such as factoring this out into a background process so
  it doesn't prevent the window from showing, and making the refresh step
  optional if we already have valid repository information, but this at
  least gets us showing the available package list from HaikuPorts.