fx_id variables in FXML JRubyFX::Controller#initialize

I assumed that variables declared with `fx_id` would be available in `initialize`, but this does not appear to be so. Is this a bug, or should I setup objects elsewhere?

Same environment as #22 

```ruby
# samples/fxml/Demo.rb

fx_id :AnchorPane
# …
def initialize(first, second)
  puts "Ruby new"
  puts "#{first} #{second}"
  p @AnchorPane # => nil
end
# …
fx_action_handler :clickbl do
  puts "Clicked Black"
  # This is the fx:id linked variable
  p @AnchorPane # => #<Java::JavafxSceneLayout::AnchorPane:0x62839dbf>
end
```