report associated pointerId for a selectionchange

[rektide]

The Pointer Events interface includes a pointerId, which identifies which pointer an input originated from. This is very useful on a workstation with multiple pointers, as without it, the page has no way to understand that two users are simultaenously working with the page.

The current selectionchange, unfortunately, has no simialr capability. When multiple users are both selecting, page has no means to make sense of the different selections that are occuring.

To fix this, pointerId should be reported in the selectionchange event.

#167 talks to some other multiple-selection issues that we face.