Move layout files to workflow settings folder

[glopesdev]

As detailed in #1955 this PR moves the .layout files to the new per-workflow settings folder which aims to declutter project working directories and is designed to be conveniently ignored by version control systems.

New example folder structure for layout files:

📦ProjectFolder
 ┣ 📂.bonsai
 ┃ ┣ 📂Workflow1
 ┃ ┃ ┗ 📜layout
 ┃ ┗ 📂Workflow2
 ┃ ┃ ┗ 📜layout
 ┣ 📜Workflow1.bonsai
 ┗ 📜Workflow2.bonsai

Further work in #1870 will allow building on this new organization to save other editor state, and possibly address some of the drawbacks outlined in #1955.

The PR also moves scanning of extension workflow files into the new Project helper class to begin consolidating project path manipulation operators in advance of refactoring towards a cross-platform visual editor.

Fixes #1955