Defer selection-triggered updates in ve.ui.Context

Move selection change handling (closing the popup if open, and updating
the context toolbar) to .afterChange(). Every time .onChange() detects
a selection change, it schedules a call to afterChange(). These calls
are batched so that multiple selection changes in the same tick cause
afterChange() to be called only once.

Deferring these updates causes them to no longer occur while a 'change'
event is being emitted. This means that if an inspectors' close handler
calls .change(), that call is now no longer nested inside another
.change() call and doesn't run afoul of any render locks set by the
caller of the outer .change().

Bug: 54675
Change-Id: Iae2f41a83b5d64251a54e42303100e84a5c25561