Do not access ~/Documents until absolutely necessary #318

Currently VMPC2000XL accesses ~/Documents even during initialization. We have to find an alternative approach:

• demo beats can be installed after the user opens the LOAD screen, and maybe with a prompt if they want that. That would also pave the road for an informative demo beats restore flow.
• vmpc.log can be put in a better dir than ~/Documents.
• MIDI control presets, we'd have to figure something out for it.
• Check what else. See mpc::Mpc::init().

One reason to fix this is to make mpc::Mpc more friendly to build in the CI/CD. We hit this case with ciwi:

This makes the pipeline hang without any feedback whatsoever.