Add first draft of a UI delegate plugin. This is functional, but
limited, and could be cleaner. Ideally work will be done upstream in
FPT to make this easier.

Provide asset browsers fo both published files and workfiles.

Workfiles are a particular pain point, since they cannot be queried from
the server, i.e. they just exist on disk in strictly structured paths.

Opening a workfile also triggers several FPT processes, and so we can't
simply return a reference to the host application. Future work will have
the manager plugin trigger these processes only once the workfile is
resolved, via some mechanism to signal to FPT that the workfile is
being resolved with the intention of opening it in a DCC (e.g. via the
Context.locale).

Provide an entity info panel showing asset information about the
selected entity. This is an interesting case since it relies on the
host asynchronously communicating the currently selected entity to the
delegated UI element.

Only read cases supported - i.e. no publishing support - at the moment.

Due to the need to import sgtk apps at runtime, many classes must be
defined inline. Alternative strategies exist, but this is left for
future work.

Since we're adapting pre-existing widgets, several overrides and fixes
must be applied. In particular, we use a "widget stash" to keep FPT
widgets alive as long as possible, to avoid issues with QThreads being
destroyed without proper cleanup.

Bump minimum version of OpenAssetIO to 1.0.0, where the UI delegation
API is available; and MediaCreation to alpha.12, where some UI-specific
traits are defined.

Signed-off-by: David Feltell <[email protected]>