Directly depend on global variables store in expression stores

[shueja]

I don't like having to do this, but we were already injecting this instance into every ExpressionStore through Variables.createExpression. (See former line 590 in ExpressionStore.tsx)

Unfortunately, undoing an operation that deletes an ExpressionStore (through deleting a waypoint, constraint, etc) does not use createExpression, so restored ExpressionStores did not get the injection. This worked in most cases because the Variables instance was injected two different ways, but only when the store was created after the Variables instance was created. In short, the existing system is fragile and puts annoying constraints on initialization order just to avoid a direct import as a way to access a global constant.