Discovered in phetsims/fourier-making-waves#250 ...

NumberSpinner does not support dynamic layout. Like NumberControl, it should probably extend Sizable and resize its NumberDisplay accordingly. (And does dynamic layout also need to be added to NumberDisplay?)

This is not blocking for phetsims/fourier-making-waves#250.

Assigning to @jonathanolson, since he added dynamic layout to most of the other common-code UI components, so is probably most familar with this.