Make the "Remap" functionality available from scripts

It would be nice to be able to "Remap" the palette from scripts or make the button visible if the script modifies the palette.

At the moment the difficult part of this one is where/how to indicate the "start point of modifications". In the UI it's from the first modification of the user palette, on scripts it might be good to have a way to indicate the start of modifications (maybe the start of a [transaction](https://github.com/aseprite/api/blob/master/api/app.md#apptransaction)?)

From: https://community.aseprite.org/t/sort-swatches-by-usage-suggestion/1641/7?u=dacap