Restore actions dialog

This PR adds in a simple restore dialog, showing a table of changes that will be performed by a `packrat::restore` call.
Calling packrat:::getRestoreActionMessages requires an updated version of packrat. So we need to: 

1. Bump the packrat version in DESCRIPTION
2. Change the required version of packrat here: https://github.com/rstudio/rstudio/blob/master/src/cpp/session/modules/SessionPackrat.cpp#L289-L290