Only re-render the selection overlay once per batch of requests

Also, decrease the maximum latency between a render request and when the
update is performed to be 0ms. The previous value of 200ms was quite
noticeable when indenting or unindenting paragraphs.
Merge pull request #329 from peitschie/selection-render-perf

Only re-render the selection overlay once per batch of requests
@returns -> @return
clickStartedWithinContainer -> clickStartedWithinCanvas
OdfCanvasAdapter -> MockOdfCanvas