Draft: 2.16.0 release branch

Branched from current master, includes !719 (merged) (and !721 (merged)). Currently building on the OBS, will do a draft release on the GitHub mirror as soon as those and my local Mac and Windows builds finish.

New features:

• interactive print dialog (AG)

• new snap to grid option (JW)

• soundfiler backport/improvements (JW)

• various GUI improvements and fixes (JW)

Bugfixes:

• fixes to sys_domicrosleep() backported from vanilla (AG)

• prevent crashes upon SIGHUP from the watchdog on Linux (AG)

• build system fixes for latest autoconf and msys2 (AG)

• general improvements in the source code (JW)