Enable users to set a default path for saving projects

When sending a model to Bambu Lab Studio from Fusion 360 the user has to navigate to a folder to save the project under ‘Save Project as’ (Ctrl+Shift+S).

Essentially, the model is being passed via command line parameters from the third-party software and it would be useful to be able to pre-populate the user’s top-level preferred path from a default defined within Studio’s Preferences, in the same way that we can already set the default ‘Downloads’ path.