Defer most Quick View processing until after delay

This is for #6538. This uses the `getHoveredEditor()` function from @SAPlayer's pull request #6571, so this is a replacement of that PR.

Details:
* only processes mousemove event in `requestAnimationFrame()`
* defers all processing until after `setTimeout()` delay
* **exception:** when Quick View is already being displayed, then processing is immediate

Results:
I did some simple profiling on Win7 using the src/extensions/default/QuickView/unittest-files/test.css file and Process Explorer. I moved mouse over the css rules in that  file at an even pace -- so that only mouse processing was executed (i.e. didn't stop long enough to display Quick View).
* **Before:** cpu averages **~2%**
* **After:** cpu averages **~1%**

cc: @peterflynn, @jasonsanjose