Fix crash when loading spectrogram. Refs #9970
Start refactoring project loading in AppWindow.

This is the first broad stroke of refactoring to use the new
TSVSerialiser class for loading/saving projects. It works, but it's far
from complete. There are a few known bugs, and a few more unknown ones
too.
Merge branch 'feature/9970_refactor_project_serialisation' into develop