Add code usage popup.

[andrechen77]

Add a the "code usage" feature from Galapagos, where selecting some code (such as a variable name) in the editor and pressing Ctrl+U creates a popup that shows all the other places in the code where that variable/whatever is used.

Right now this has to be done completely outside of the editor, but if we're going to renovate the editor anyway, it would make more sense to move this feature over. This is because the editor already has facilities to create similar popups (code completion), plus the fact that using some form of AST would make the feature far superior to the textual-search currently used.