Issue 2717: Implement \single for converting overrides to tweaks

Issue 2717: Implement \single for converting overrides to tweaks

\single, like \once, can be applied to music consisting of one or more
overrides.  It translates such an override set, for example
\easyHeadsOn, into a tweak on the following music item, so you can use
it like

{ < \single\easyHeadsOn c' g' >2 q }

inside of a chord and have it only affect a single notehead of the
chord when \once\easyHeadsOn would affect all noteheads.

In that manner, most overrides defined in ly/property-init.ly
are made available as tweaks.