Backspace always trimmed the last character and typing always appended,
ignoring any selection. So "select-all, then Backspace to clear" left
all but one character, and typing over a selection accumulated text
instead of replacing it.
Backspace now deletes the selected range when one is set, and typed
text replaces it, matching the browser. selectionStart is null during
ordinary typing, so the previous append/trim behavior is preserved when
no selection is tracked.
Pairs with the triple-click select-all in #320 to make the common
"triple-click, Backspace, retype" clear pattern work end to end. Also
fixes the same pattern when a page sets the selection via select() or
setSelectionRange().
Gabriel
0913be86dc