Wiring for Custom Datadir

[D33r-Gee]

This pull request builds upon previous issues (#390, #392, and #397) by re-introducing the functionality for users to specify a custom data directory (datadir) during the onboarding process.

The majority of changes in qml/bitcoin.cpp are mainly splitting the onboarding and node creation so that the user can set their preferred datatdir it's inspired by the qt/bitcoin.cpp however it uses the anonymous namespace instead of using a class. The intention came from feedback on https://github.com/bitcoin-core/gui-qml/pull/390

In the QML pages, the node_model context property is no longer been initialized during onboarding. We're now using options_model instead. This means the code has been updated to use the options_model for settings that were previously set with the node_model context property.

Support for the Android version is being added to this PR.