MacroExpander: Allow registering local expansions

This allows a MacroExpander also to describe an expansion it can
do which was restricted to the global VariableManager only.

The global is now just a thin (unneeded) wrapper about new "standard"
functionality.

Change-Id: Ida7ca70cf3d319eae4220ea8d12f3dd1c0d4042c
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>