Clarify association between a selection and its range

[rniwa]

"Once a selection is associated with a given range, it must continue to be associated with that same range until this specification requires otherwise."

This paragraph above is vague. It needs to be replaced by detailed conformance requirements saying exactly what to do for particular keystrokes, like we have for backspace/delete/etc.

[yoichio]

Current spec states:
"Each selection can be associated with a single range. When there is no range associated with the selection, the selection is empty. The selection must be initially empty."
but per WICG/webcomponents#79, selection should be associated with a composition range so that
selection can represent range-over-shadow and calling SetBaseAndExtent with shadow nodes sets such selection simply w/o any adjustment.

Concerns.
Selection is associated with DOM Range, witch adjusts itself when DOM tree changes but a composition range might not or it is not speced.
getRangeAt() needs some adjustment to return DOM Range from the composition range.

[rniwa]

Yeah, I think we need to change the internal model of selection to be some kind of composed range from which Range and composed range exposed to a script is computed.