Selectmenu: Document generated ids

[gabrielschulhof]

Given a selectmenu's id, we derive ids for the various elements of its enhanced version:

button

<selectmenu-id>-button
This is both for custom and native selectmenu. The rest are for custom only.

popup

<selectmenu-id>-listbox

screen

<selectmenu-id>-listbox-screen

container

<selectmenu-id>-listbox-popup

listview

<selectmenu-id>-menu

We should document this