Add very basic JACK transport support

This is just enough to connect Synfig to a running JACK transport and
control it from the buttons in the canvas view. It only works one way,
Synfig controls the JACK transport.

Also add jackaudio as a build input for the nix expression, to make it
easy for end users and developers to test it out.
JACK: add option to enable, but disable by default
Fix JACK configuration. Enable JACK by default (temporary).
full JACK support
Autodetect JACK presence and make it optional.
Add stub for JACK button icon
Move JACK button into different place
Don't show JACK button if JACK is disabled.

Also, add environment variable SYNFIG_DISABLE_JACK to explicitly disable JACK.
It's needed for packaging routines.
Linux package: Add JACK support
Merge branch 'jack-transport'