Minimal menu for USB read/write operations

[Dewb]

What does this PR do?

Adds a minimal menu for USB operations. This feature design was suggested by @beels as a minimal useful gesture towards his
WIP usb-disk-menu branch.

How should this be manually tested?

Insert a USB stick. You should see a menu with four options. Turn the param knob to highlight an option and press the front button to select it. EXIT will return to live mode without doing anything. WRITE TO USB will copy scenes from flash to the tt00s.txt files. READ FROM USB will copy the contents of the tt00.txt files to flash memory. DO BOTH will perform both a write and then a read, the previous behavior.

Any background context you want to provide?

From Discord discussion, there are a couple motivations here:

• providing a failsafe to avoid inadvertent usb operations if an inserted USB drive is physically touched in such a way that it retriggers the MSC connection event.
• making a minimally useful gesture in this direction to reduce the activation energy for continuing to enhance the usb menu in future releases.

If the related Github issues aren't referenced in your commits, please link to them here.

#81

I have,

• updated CHANGELOG.md and whats_new.md
• (n/a) updated the documentation
• (n/a) updated help_mode.c (if applicable)

[Dewb]

recommend viewing the diff in "hide whitespace" mode.

[tehn]

thanks! this is a huge UI improvement.

code looks good, did not test on hardware.

[scanner-darkly]

looks good!