[UI] Feature: Functions for basic questions

[GB609]

This is a massive enhancement and partial re-implementation of the user-interface.shl library which provides utilities to ask users for input.

1. Introduced an 'api' file for documentation and verification purposes. Might not be necessary in production later on.
2. All existing ask... functions were split into a public-facing part and backend-specific implementations.
3. Provide several generic utility functions for use from within user-interface.shl and its submodules.
4. Provided implementations to ask for files (with extension check) and directories.

closes: #143