Add editing pane for NuBlocks to Nutron

[itfrombit]

When an NuBlock is selected in the object browser, display the nubile'd source code in an editor pane. An "Update" button should save the updated version of the block and refresh the object viewer.

[ksjogo]

I implemented a kind of that behaviour in my fork: https://github.com/ksjogo/nutron
It is not only for blocks, but macros and nubased instance methods as well (but there is no button at the moment, a cmd+s must do).