It has been a year or two since I touched the Inkscape codebase but have recently been introduced to the idea of a "Screaming architecture by Robert C. Martin/Uncle Bob" [1, 2] in other development efforts and started to practice it. It might not be doable right now for Inkscape but goes along the idea that we want WITH_UI flag in the build system (be able to build headless inkscape that is). Also it should make it easier for new developers to understand the codebase just by looking at the file/directory structure.
### Inside "src" folder
There is nothing in the directory structure that clearly shows that this is an awesome SVG application.
1. What is the difference between "display" and "ui"?
2. Is "ui" a gui or also command line?
3. "Manipulation", it is impossible to understand what it is being manipulated with just a one-layer folder structure.