Spiff up the demo script a bit.

- Use nicer formatting for the code
- Demo a reverse-order incremental clearing of the screen
Incorporate proper wait times into LCDSysInfo to speed up drawing

(The device gets slow if it draws and handles USB traffic at the same time
and it's easy for users to forget to wait in their own program. It already
waits in some commands, so we might as well stay consistent.)
Tidy image-loop.py and show-*.py and use the new large_image_indexes list
Minor formatting adjustment in info.py