Refactor and delete src/app/gui folder

[Raruto]

The GUI term is ambiguous and too generic (javascript itself is a frontend language), in the next versions find and relocate the last files left here src/app/gui and then definitively deprecate that folder.

.
└── src/
    ├── app/
    │   └── gui
    ├── components
    ├── services
    └── store

[Raruto]

Link here a related image from #99