Prevent creating a global variable with the same name as a local one

This also adds a dialog box explaining why the variable could not be created.

Fixes #119.
If you change this to use lookup early and create later instead of using lookupOrCreate, you could provide the same feedback in the case where you try to create a sprite variable with the same name as an existing global variable. Currently that case offers no feedback: it just silently refuses to create a new variable due to lookupOrCreate.