Feature description

From what I understood by reading
https://github.com/termux/termux-app/blob/master/app/src/main/java/com/termux/app/ExtraKeysView.java
only the — and / keys on the termux touch keyboard have alternative keys when swiping them up. I think it would be really nice to make this behavior customizable in the termux.properties file, so we can choose which keys will have an alternative key, and which alternative key it would be.

Maybe it would also be nice to add a swipe down gesture too. This way, each key would have 3 different possibilities, thus substantially reducing the necessity of multiple extra rows.