Feature Request - ComboBox-style widget with embedded TreeView

**[Original report](https://bitbucket.org/controlsfx/controlsfx/issue/235) by Christian Michelsen (Bitbucket: [flinkgutt](https://bitbucket.org/flinkgutt), GitHub: [flinkgutt](https://github.com/flinkgutt)).**

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* The ComboBox should display one tree (expanded or not).
* On selection the selected node is to be displayed in the regular manner of a combobox.
* Should (probably) support expanding tree without closing the "popover", see point 1.
* Tree would hopefully be just like any other TreeView in what it supports, might even be cool if it could be swapped out with a CheckBoxTree.
* Support for having some nodes be "unselectable", and if "selected" the popover doesn't close.

As to how to actually implement this I've drawn somewhat of a blank, just tossing in a TreeView into a ComboBox isn't really working all that well, for me at least.

This could I suppose be solved with some "container" other than ComboBox.