[JRI] Intercept View(x) calls

[laeubi]

For example in RStudio if one types View(x) then not the standard R-View is opende but the variable is displayed inline in RStudio. I'd like to support something similar, thus it would be cool to have a method in RMainLoopCallbacks that allows for that, e.g. the method could return true/false, and if returning false then not the default view action is triggered.