Re-evaluate how registry keys are written for RRO.

[j-martens]

Re-evaluate how registry keys are written.

R and RRO uses the same key (IIUC) - so when R is installed after installing RRO, the registry keys disappear. Then, to get RStudio to recognize RRO, one must manually point to the file location where RRO is installed. If we design the key to look slightly different, then GNU R and RRO will not interfere with one another.

RStudio in fact picks up "all" installations of R, including all old installations. This is why R and RRE easily sits side by side. Somehow RStudio uses the last installed version as the default. However, RRO breaks this paradigm, since it doesn't show up in the list of installed versions.

We want to find a way to have both RRO and R both show up. This solution should consider how it might affect RExcel.

For suggestions, screenshots, and other ideas, see here: #174