Add miniscreen app for onboarding

[jcapona]

The other side of pi-top/pi-top-4-Miniscreen#139

Note: this is using a placeholder GIF for AP mode. This PR is blocked until we have the assets (pi-top/pi-top-4-Miniscreen#136)

[m-roberts]

From what I can tell, this still need first boot logic to be ported. We should replace the FS expand breadcrumb with something more robust. One possible option would be to check the filesystem size, though I'm sure there are other ways to detect.

[m-roberts]

We should ensure that pt-sys-oled is not running before starting.

If there is any user code using the display and we do nothing else, the entire application will lock up because the miniscreen is considered to already be in use (See pi-top/pi-top-Python-SDK#386 for a fix for this).

Therefore, for safety, we should include the PT_MINISCREEN_SYSTEM environment variable, which prevents locking but is vulnerable to other code taking the miniscreen. This does not affect the primary pi-topOS use cases of concern.