ListComboBox feature list

[hamen]

• When the text field is in focus and receiving text, if I open the dropdown I want to still be able to type in the text field. In this scenario, ⬆️⬇️ will cycle through the elements in the dropdown, while ➡️⬅️ will move the caret in the text field.
• Selecting an element in the dropdown using the ⬆️⬇️ will replace the content of the text field.
• Typing the exact value of one of the elements in the dropdown and pressing Enter, the element in the list must be selected.
• Nice to have: popupWidth for the user to specify a custom popup width.

[rock3r]

As context to this, we're going to move ComboBox's API to two forms:

1. A bring-your-own-popup-content ComboBox which does none of the things above. The current Dropdown behaviour can be implemented with this, plus a Menu (this requires Split out popup window background from menus #528 and Make Dropdown have generic content popup #529)
2. A turn-key, works-like-Swing ListComboBox, which takes in a (Selectable)LazyListScope for the dropdown content, handles the list by itself, and provides all the above behaviours that match Swing, out of the box

Note: the first two items in the OP require the popup and its contents not to be focusable, and moving the keyboard events handling to the BTF instead (which will retain focus when the popup opens).