Much like the themable selects and checkboxes, number spinners
should also be themable.
Standard number spinners are not very customizable. We should use
existing buttons and fonts to add their functionality to allow a
richer experience if desired downstream.
An example of how to customize the spinner was placed in Material.
The example shows how to use flexbox to change layout type from
column to row, change icon order, and how to override the icons.
'autocomplete' needs to be false on this new element, otherwise
the browser will retain and load the last value without actually
triggering any JavaScript events by which we can key on and update
the state of the spinner buttons.
Change-Id: Ifd266cd515a903841e2d28e2f4731879116e3513
Closes-bug: #1598311
ba35724341