Selectmenu: Support `width: false` and default to 14em

Selectmenu: Support `width: false` and default to 14em

`width: null` still matches the width of the original element.
`width: false` prevents an inline style from being set for the width. This
makes it easy to set the width via a stylesheet and allows the use of any
unit for setting the width, such as the new default of `14em`.

Fixes #11198
Closes gh-1467