Add file I/O to toolbar

### Problem to be solved

We currently have a somewhat awkward setup with the Upload audio button being put with Audio setup, Get effects and, in the future, the mixer. Having ads for these services always-visible and impossible to hide is neither beneficial for Audacity's brand image, nor beneficial for sales as people with a strong enough animosity towards these features to go out of their way to hide them would not use them anyway.

At the same time, the classic New/Open/Save/Import/Export buttons, found in loads of (legacy) software, are nowhere to be found in Audacity, and historically haven't been either.

### Your idea

Given that we now have a configurable toolbar, we should consider adding fileIO actions optionally to it. This is going to be especially helpful for powerusers making strong use of #762 (assuming that gets implemented).

### Prior art

https://github.com/audacity/audacity/pull/11186 has a POC with various additional I/O buttons in the toolbar.

### Additional context

* #11230

Start by reviewing the proof of concept in pull request #11186 and the configurable toolbar context referenced by issue #762. Clarify the intended optional file I/O actions, including New, Open, Save, Import, and Export, and define completion as those actions being available through the toolbar without making the service controls permanently visible.

Written by the indexing model from the issue text.