Allow disabled buttons to have no pointer-events

Before, disabled paper-icon-button and core-icon-button still received
pointer-events. See Gist https://gist.github.com/MetaMemoryT/17c216c4a45add089f9e#file-buttons-in-core-toolbar-disabled-noclick-html for a demonstration.  The Gist also includes some style rules to work around the existing implementation.