Support for callback timing and general graph improvements #223

We should lay the foundations for supporting the features added by plotly/dash#1179 as a follow-up to plotly/dash#1176, which will permit timing of callback execution and add the ability to reorganize existing nodes on the graph when the dev tools UI is opened (among other improvements):

The changes required are relatively trivial and could be added concurrently with the upcoming pattern matching callbacks PR for our next CRAN release.