feat: Keyboard list navigation improved

[micz]

See #209.
Not including aria elements as agreed.

[micz]

At the moment, with each update of the list, if the selected item still exists, it remains selected. Using this feature daily, I was wondering whether it might be better to add an option to always select the first item with each update. What do you think?

[micz]

I changed the code in this way:

• Using the observer to select the first line when opening the popup, then the observer is disconnected.
• Selecting the first line when searching directly in the repopulate() method.

The question in #215 (comment) is still valid.