Merge base pip packaging

[266-750Balloons]

TODO before you merge to main: Merge my sdec pip pull request, as this branch is set to reference main SDEC as a pip dependency when main branch SDEC is not yet a pip package.

This pull request merges the base support for the liquid_engine_gui as a pip package. Note that it majorly reorganizes the directory structure as follows:

• All source code for the GUI has been moved into a directory called liquid_engine_gui.
• The old prototype code has been moved into the extras folder.
• The images folder has been moved under the source directory to allow the liquid gui module to access image assets.

Make sure to update the version in pyproject.toml to 2024.9.0, removing the branch indicator.