Add a global command to move braille to the current focus (#5491)
* On a pending caret move if there is no cursor, try to scroll the braille display to the selection.
* Move the marking of a selection to braille.Region.
* Add Region.brailleSelectionStart/End.
* If there is a selection but not a cursor, e.g. in a TextInfoRegion, scroll the braille display to the selection start.
* Add a global command to move braille to the focus, and the caret or cursor within the focus object if possible.
* If tethered to review, move braille to the focus object.
* Otherwise, move braille to the main buffer's last region, which should contain the focus. If it has a cursor/caret, scroll to it. If instead it has a selection, scroll to that.
* Check that there are actually some regions before selecting one.
* If no cursor is found, try scrolling to the selection. Requires #5410.
* Make the script documentation a bit clearer.
* Sync submodules.
