Notify the text field selection changed in the next run loop.

We ran into a case where the input element's selection had not changed
yet when mouseUp was called. This lead to text being incorrectly wiped
out when the previous selection spanned multiple characters.
Fixed an issue with observing the `selection` property of `SC.TextFieldView`. Since the `input` element doesn't update its `selectionStart` and `selectionEnd` values until after the event execution completes, the mouse and keypress event handlers that affect the selection must wait until after the run loop before notifying that the selection property has changed.