Expose init executor instance

[promag]

This PR exposes InitExecutor instance to QML so that we can:

• show the window as early as possible
• display loading feedback on that window
• connect/bind in QML to the relevant object's properties/signals to update the UI

The InitExecutor is then kept in the main GUI thread (otherwise QML engine complains) and the instance is exposed in the root engine context.

This change also removes the "proxy role" of NodeModel. Further simplifications/improvements will follow, but I'd like to present this approach first before going forward.

[promag]

Related bitcoin-core/gui#434

[hebasto]

Concept ACK.

[hebasto]

Concept ACK.

To elaborate, I'm "Concept ACK" on:

• show the window as early as possible

• display loading feedback on that window

But I'm not sure it is required to expose an InitExecutor instance to achieve these goals.

A relevant discussion in bitcoin-core/gui#434:

• Keep InitExecutor in main gui thread gui#434 (comment)
• Keep InitExecutor in main gui thread gui#434 (comment)

On of the main goals of starting this project is involving Bitcoin Design community in the development process from the day 0. And to achieve it, keeping QML files as simple as possible is important for the project.

As were mentioned in #31, with the current approach:

• QML files are easy to read, reason about, and modify. Designers could easy work on them

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Anyway, as bitcoin-core/gui#434 is merged, this PR requires rebasing on top of the updated sync with the main repo that is currently blocked by #41.

[promag]

Currently the designer is already exposed to nodeModel and nodeModel.startNodeInitializionThread(). And eventually it will have to be exposed to more stuff since that's the way QML works.

This change at least removes the wiring from InitExecutor and NodeModel.

Follow-ups can improve how QML deals with the application state.

[hebasto]

Currently the designer is already exposed to nodeModel and nodeModel.startNodeInitializionThread().

It is all about the same object :)

And eventually it will have to be exposed to more stuff since that's the way QML works.

"more stuff" could remain the same single nodeModel object.

This change at least removes the wiring from InitExecutor and NodeModel.

But it exposes another object into QML code.

Having the only object, which is exposed to the QML, has a simple benefit of a guarantee that its internal state, i.e. internal state of the C++ backend, is consistent at anytime.