EditorClientHaiku: check for a selection the right way.

Instead of checking if the selection "starts" somewhere, check if it is
a range-selection, in which case the arrow-keys are used to edit it (not
for scrolling the page).

I'm not sure supporting keyboard-editing of selection this way is
useful, and there may be other similar problems with other keys, added a
TODO about that.

Fixes #10621.