ComboBoxAttachment: changing value to visual order correlation

Yep, the current incarnation of the ComboBoxAttacment is severely restricted because of that strange design as I have noted here:

I needed a more flexible ComboBoxAttachment and so I have created my own version. If you don’t mind using hacked versions of some Juce files You will find that it allows to easily create a Combobox with all the features outlined above.