I've released mkvtoolnix v7.2.0. It fixes a couple of small bugs. Furthermore it fixes compilation with the just-released Boost v1.56.0 which is source-incompatible to earlier releases in certain ways.
Behind the scenes a lot of work has gone into the re-write of mmg called MKVToolNix GUI. This release is the first one to feature a preview of that new GUI, but only in the Windows binaries (users of other operating systems will have to compile the package themselves if they want to run the new GUI). More details are available on the MKVToolNix GUI status page.
For packagers: note that the REAMDE file has been converted to Markdown format. Therefore it has been renamed to README.md.