Add <Transform> support to <Connection> blocks in the skin.

Adds a general ValueTransformer class that does simple, invertible
transformations on numbers. We could re-use this in other places, e.g. MIDI
mappings to allow customizability to advanced users.
Merge branch 'master' into transform

Conflicts:
	src/skin/legacyskinparser.cpp
Add arm64 and ppc64el to the machine whitelist.
Merge branch 'master' into transform

Conflicts:
	src/skin/legacyskinparser.cpp
	src/widget/controlwidgetconnection.cpp
	src/widget/controlwidgetconnection.h
Fix recent build copying on OS X.