Add a Snowflake helper

[cgueret]

Is your feature request related to a problem? Please describe.
Some authors will want to use the Snowflake method to design their novel. The tool can support some aspects of it but it requires clicking around

Describe the solution you'd like
We could have in the planning tools a dedicated multi-step wizard guiding users throug the SnowFlake approach. Each of the step would be a dedicated NavigationPage. We could maybe even introduce a timer to respect the suggested time objectives for the writing.