RTA Treat characters from supplementary unicode planes as unit

[tonydroy]

The RTA does a good job displaying arbitrary Unicode characters. It would be good if it could treat characters from the Supplementary Unicode Planes that are coded as a pair, as a single unit for purposes of cursor movement, deleting and such - rather than as distinct characters. There is no indication that this is part of the gluonhq project. Still the issue is located within the RTA code. And I think this must be a reasonably natural enhancement to the RTA, as it already treats high-value emoji points as a unit.