Fix selection of unmarked RtL text

Fix selection of unmarked RtL text

With Qt 5.11 at least, RtL text will be drawn RtL even when the
(undocumented) flag Qt::TextForceLeftToRight is applied to the
QTextLayout object. This creates selection issues for Hebrew text
marked as English.

The solution is to do the same as in breakAt_helper, that is prepend
the string with a direction override unicode character.

Doing this requires to introduce a TEXTLAYOUT_OFFSET constant that has
to be used wisely to account for this extra character.

Fixes bug #11284.