ENYO-1455: Accommodate mouse events on scroll controls
ENYO-1455: Accommodate mouse events on scroll controls

enyo/Scrollable has an experimental feature to suppress mouse
events during scrolling, which can improve scrolling performance.

However, some scroll controls may not behave properly with mouse
events suppressed; this was the case with moonstone/ScrollControls.

We provide a mechanism that allows scroll controls to selectively
disable mouse-event suppression as needed.

In doing so, also refactored the code a bit and made
`suppressMouseEvents` an explicit option (vs. enabling / disabling
it based on whether the `touch` option was selected).

Enyo-DCO-1.1-Signed-Off-By: Gray Norton (gray.norton@lge.com)

PKRoma/enyo,enyojs/enyo,PKRoma/enyo,enyojs/enyo