Clear the selection after deleting a character.
Not doing so can result in out-of-bounds reads in
EditTextWidget::drawWidget(), where _selCaretPos and _selOffset are
used as offsets, and to a failed assertion when calling
EditableWidget::defaultKeyDownHandler():
gui/widgets/editable.cpp:566:
bool GUI::EditableWidget::setCaretPos(int):
Assertion `newPos >= 0 && newPos <= (int)_editString.size()' failed.
Fixes#14584
4867e14d60