rqt_runtime_monitor fixes

The rqt_runtime_monitor, as released in groovy/hydro, isn't usable for diagnostics introspection.

* Selecting any item in the left window (to see details) will cause the display to crash
* Not having any items selected will cause an error message to print about "self._current_info"
* After an item is selected, the scroll state is reset for every update

The attached commit fixes these issues. Tested on large diagnostic dataset on Groovy.

Thanks for merging this in, it will really help visualizing diagnostics.